B1-5a Digital Techniques I SR

Student Resource

Subject B1-5a Digital Techniques 1

Copyright © 2008 Aviation Australia All rights reserved. No part of this document may be reproduced, transferred, sold, or otherwise disposed of, without the written permission of Aviation Australia.

AA Form TO-18 B1-5 Digital Techniques I

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CONTENTS Page Definitions

2

Study Resources

3

Introduction

5

Numbering Systems

5.2-1

Data Conversion

5.3-1

Data Buses

5.4-1

Logic Circuits

5.5-1

Basic Computer Structure

5.6-1

Fibre Optics

5.10-1

Electronic Displays

5.11-1

Electrostatic Sensitive Devices

5.12-1

Issue: B – January 2008

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DEFINITIONS Define 

To describe the nature or basic qualities of.



To state the precise meaning of (a word or sense of a word).

State 

Specify in words or writing.



To set forth in words; declare.

Identify 

To establish the identity of.

List 

Itemise.

Describe 

Represent in words enabling hearer or reader to form an idea of an object or process.



To tell the facts, details, or particulars of something verbally or in writing.

Explain 

Make known in detail.



Offer reason for cause and effect.


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STUDY RESOURCES Jeppesen General Jeppesen Airframe AC 43.13-1B/ AC 43.13-2A Combined – Aircraft Inspection and Repair Student Handout – Subject B1-5a


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INTRODUCTION The purpose of this subject is to allow you to gain knowledge of the application of digital and electronic systems used in aircraft. On completion of the following topics you will be able to: Topic 5.2

Numbering Systems Define the following numbering systems and perform conversions between them:  Decimal  Binary  Octal  Hexadecimal

Topic 5.3

Data Conversion Define the operation of conversion between analogue and digital data using the following:  Analogue to Digital converters (AD Converters)  Digital to Analogue converters (DA converters) List the inputs and outputs of AD and DA converters and define the operation and limitations of various types.

Topic 5.4

Data Buses Describe the operation of data bus systems used in aircraft systems. Describe the properties of data bus communication protocols including:  ARINC 429  ARINC 629  MIL-STD 1553

Topic 5.5.1

Logic Circuits Identify common logic gate symbols. Describe the operation of common logic gates, state their applicable truth tables and describe equivalent circuits. Describe applications of logic circuits used in aircraft systems.


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Topic 5.6.1

Basic Computer Structure Describe the following computer terminology:  Bit  Byte  Software  Hardware  CPU (Central Processor Unit)  IC (Integrated Circuit)  RAM (Random Access Memory)  ROM (Read Only Memory)  PROM (Programmable Read Only Memory) Describe the application of computer technology in aircraft systems.

Topic 5.10

Fibre Optics Define the advantages and disadvantages of fibre optic data transmission over electrical wire propagation. Define the operation of a fibre optic data bus and list fibre optic related terms. Define how terminations are performed on fibre optic cable. Identify the following and state their purpose in a fibre optic system:  Couplers  Control terminals  Remote terminals List aircraft systems which utilise the application of fibre optics.

Topic 5.11

Electronic Displays Describe the principles of operation of the following electronic displays used in aircraft:  Cathode ray tubes  Light emitting diodes  Liquid crystal displays

Topic 5.12

Electrostatic Sensitive Devices Describe the special handling requirements for components sensitive to electrostatic discharge. Identify risks to electrostatic sensitive devices and describe possible damage to components. Identify component and personal anti-static protection devices.


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TOPIC 5.2 – NUMBERING SYSTEMS Computers are now employed wherever repeated calculations or the processing of huge amounts of data is needed. The greatest applications are found in aviation, military, scientific, and commercial fields. They have applications that range from mail sorting, through engineering design, to navigation around the globe. The advantages of digital computers include speed, accuracy, and manpower savings. Often computers are able to take over routine jobs and release personnel for more important work, work that cannot be handled by a computer. People and computers do not normally speak the same language. Methods of translating information into forms that are understandable and usable to both are necessary. Humans generally speak in words and numbers expressed in the decimal number system, while computers only understand coded electronic pulses that represent digital information. This lesson will cover number systems in general and about binary, octal, and hexadecimal (which we will refer to as hex) number systems specifically. Methods for converting numbers in the binary, octal, and hex systems to equivalent numbers in the decimal system (and vice versa) will also be described. You will see that these number systems can be easily converted to the electronic signals necessary for digital equipment.

Types Of Number Systems Until now, you have probably used only one number system, the decimal system. You may also be familiar with the Roman numeral system, even though you seldom use it.

Numbering Systems Numbering systems have certain things in common. These common terms will be defined using the decimal system as our base. Each term will be related to each number system as that number system is introduced. Each of the number systems covered is built around the following components: the UNIT, NUMBER, and BASE.

Unit and Number The terms unit and number when used with the decimal system are almost self-explanatory. By definition the unit is a single object; that is, an apple, a dollar, a day. A number is a symbol representing a unit or a quantity. The figures 0, 1, 2, and 3 through 9 are the symbols used in the decimal system. These symbols are called Arabic numerals or figures. Other symbols may be used for different number systems. For example, the symbols used with the Roman numeral system are letters, V is the symbol for 5, X for 10, M for 1,000, and so forth. We will use Arabic numerals and letters in the number system discussions.

Base The base of a number system tells you the number of symbols used in that system. The base of any system is always expressed in decimal numbers. The base of the decimal system is 10. This means there are 10 symbols – 0, 1, 2, 3, 4, 5, 6, 7, 8, and 9 – used in the system. A number system using three symbols – 0, 1, and 2 – would be base 3; four symbols would be base 4; and so on. Remember to count the zero or the symbol used for zero when determining the number of symbols used in a number system. The base of a number system is indicated by a subscript (decimal number) following the value of the number. The following are examples of numerical values in different bases with the subscript to indicate the base. When indicating numbering systems, if there is any chance of misunderstanding the number base in use, the number should be subscripted with its base e.g. 10102 is 1010 binary, 101010 is 1010 decimal and 101016 is hexadecimal, all of which indicate totally different numbers.

Issue B: January 2008

Training Material Only Revision 2

Topic 5.2: Numbering Systems Page 1 of 14


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You should notice the highest value symbol used in a number system is always one less than the base of the system. In base 10 the largest value symbol possible is 9; in base 5 it is 4; in base 3 it is 2. 0 1 2 3 4 5 6 7 8 9

Decimal Numbering System

I II III IV V VI VII VIII IX X

Roman Numerals – also base 10

Both systems represent a value even though the symbols are different: IV = 4

VIII = 8

9 = IX

A Unit is a single object or quantity, eg a dollar, a Litre of fuel, a day A numeral is the symbol that represents a unit or quantity, eg 5 = ♣ ♣ ♣ ♣ ♣ Base of a numbering system indicates amount of symbols used in the system Base 10,

10 symbols:

0 1 2 3 4 5 6 7 8 9

Base 8,

8 symbols:

0 1 2 3 4 5 6 7

Base 2,

2 symbols:

0 1

Base 16,

16 symbols:

0 1 2 3 4 5 6 7 8 9 A B C D E F

Base of a number system is indicated by subscript: 7 59210 6 3278 10 100 1012 3AG 57F16

Positional Notation And Zero You must observe two principles when counting or writing quantities or numerical values. They are the POSITIONAL NOTATION and the ZERO principles. Positional notation is a system where the value of a number is defined not only by the symbol but by the symbol’s position. Let’s examine the decimal (base 10) value of 427. You know from experience that this value is four hundred twenty-seven. Now examine the position of each number: If 427 is the quantity you wish to express, then each number must be in the position shown. If you exchange the positions of the 2 and the 7, then you change the value. Each position in the positional notation system represents a power of the base, or radix. A POWER is the number of times a base is multiplied by itself. The power is written above and to the right of the base and is called an EXPONENT. In the bottom example the number 6348 is equal to 41210. Where the 2 in the upper number indicated 2 times 20, the 3 in the lower number equals 3 times 8, etc for the hundreds columns. We have just converted the octal number 6348 into a decimal number, and will explain how to do this later in the lesson Just as important as positional notation is the use of the zero. The placement of the zero in a number can have quite an effect on the value being represented. Sometimes a position in a number does not have a value between 1 and 9. Consider how this would affect your next pay check. If you were expecting a check for $605.47, you wouldn’t want it to be $65.47. Leaving out the zero in this case means a difference of $540.00. In the number 605.47,

the zero indicates that there are no tens, and is a very important numeral to include to indicate a value.





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4 2 7 This numeral represents 7 units – 7 x 100 Base 10 numbering system

This numeral represents 20 units – 2 x 101 This numeral represents 400 units – 4 x 102

6 3 4 This numeral represents 410 units – 4 x 80 Base 8 numbering system

This numeral represents 2410 units – 3 x 81 This numeral represents 38410 units – 6 x 82

If the position of a numeral is changed the overall value represented is also changed

Numbering Systems Base 10 system – Decimal – universal method of counting and recording values Base 2 system – Binary or Digital – computers perform all calculations and processes in binary – consequence of transistor state: ON or OFF Base 8 system – Octal – Digital numbers are readily converted to octal and octal is far more easily read and understood than digital. An octal numeral represents 3 binary bits Base 16 system – Hexadecimal – Similar to octal in respect to ease of conversion from binary. A hexadecimal numeral represents 4 binary bits The decimal numbering system is of course the system used universally. It is based on the number 10, probably because we have 10 fingers. The decimal numbering system used by humans, in which numbers have a base of 10, appears to have originated over 5000 years ago in India, and is believed to have its origins in the fact that we have 10 digits (fingers/thumbs). Unlike analog which uses continuously changing values, digital uses discreet numerical values to represent waveforms. Those values are not represented by the familiar decimal numbering system that we use in our daily lives but rather, the binary number system. To represent 10 different values a computer would need to incorporated 10 levels, eg brightness of lights, voltages, clock-pulses, etc. As we all know, a computer only works in digital, zero’s and ones. This is easily represented, something is either “on” (1) or “off” (0). Binary/digital devised by mathematician/philosopher, Gottfried Wilhelm von Leibnitz in 1679. So when you type a decimal number into your calculator, it converts it to digital, performs the calculation, then converts the answer back into decimal for display. In order to comprehend how a computer functions you must have an understanding of the different numbering systems. In addition to the decimal system we will cover Base 2 (Binary), Base 8 (Octal) and Base 16 (Hexadecimal).





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As mentioned before, with one digit (a bit, short for binary digit), there are two possible values. With two bits, there are four possible values. With 3 bits, there are 8 possible values. With 4 bits (a nibble) there are 16 possible values and so on... Bit

1

Nibble

0101

Byte

0000 0101

Word

0000 0000 0000 0101

Binary Numbering System Decimal: 104 = 10000

103 = 1000

Binary: 26 = 64 25 = 32

24 = 16

102 = 100 101 = 10 23 = 8

22 = 4 21 = 2

100 = 1 20 = 1

100012 (Binary Number) = 1710 (Decimal) This is determined by using the Binary Number Truth Table 27

26

25

24

23

22

21

20

128

64

32

16

8

4

2

1

1

0

0

0

1

16 + 0 + 0 + 0 + 1 = 1710 To convert 110011012 to decimal 27

26

25

24

23

22

21

20

128

64

32

16

8

4

2

1

1

1

0

0

1

1

0

1

128 + 64 + 0 + 0 + 8 + 4 + 0 + 1 = 20510 This is the base 2 system so only two symbols, 0 and 1, are used. When you are working with the decimal system, you normally don't use the subscript. Now that you will be working with number systems other than the decimal system, it is important that you use the subscript so that you are sure of the system being referred to. As in the decimal number system, the principle of positional notation applies to the binary number system. The decimal system uses powers of 10 to determine the value of a position. The binary system uses powers of 2 to determine the value of a position. The truth table provides a ready reckoner to convert from binary to decimal. Because each position is the base of the number lifted to a power, eg 21, 22, 23, 24, etc we simply calculate the values and write them across a page. Below the values record the number to be converted and then add each of the values which has a 1 in its column (as displayed above)

Decimal To Binary Conversion There are two basic ways to do decimal-binary conversion. You may find one or the other easier to understand and use.





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Method 1 - Division: This method repeatedly divides a decimal number by 2 and records the quotient and remainder. The remainder digits (a sequence of zeros and ones) form the binary equivalent. Find the binary equivalent of the decimal number 1910 The digits in the remainder column form the binary equivalent of the decimal number. Start with the bottom (last) remainder digit and write the binary number from left to right:1 0 0 1 12 To convert a Decimal number to Binary there are two methods:

• •

Division Subtraction

Division Method

To convert 1910 to

To convert 5210 to

19 ÷ 2 = 9

Rem 1

52 ÷ 2 = 26

Rem 0

9÷2= 4

Rem 1

26 ÷ 2 = 13

Rem 0

4÷2= 2

Rem 0

13 ÷ 2 = 6

Rem 1

2÷2= 1

Rem 0

6÷2= 3

Rem 0

1÷ 2 = 0

Rem 1

3÷2= 1

Rem 1

1÷2= 0

Rem 1

1910 = 1 0 0 1 12

5210 = 1 1 0 1 0 02

Method 2 - Subtraction: The subtraction method involves repeatedly subtracting powers of 2 from the decimal number. You need to have a list of powers of 2 - up to the highest power of 2 that is less than or equal to the number you are converting. In our case we are converting decimal 11 - the largest power of two less than or equal to 11 is 8 (2 to the third ). To convert 7510 to binary Refer to the binary truth table 27

26

25

24

23

22

21

20

128

64

32

16

8

4

2

1

1

1

1

1

1 1

1 0

0

1

0

Start with the largest power of 2 which can be subtracted from the number to be converted Annotate a 1 in the column under 64 and calculate 75 – 64 = 11 Now what is the highest power of 2 subtractable from 11 …………it’s 8 Annotate a 1 in the column under 8 and calculate 11 – 8 = 3 Annotate a 1 in the column under 2 and calculate 3 – 2 = 1 Annotate a 1 in the column under 1 which leaves 1 – 1 = 0 Now fill in all the spaces in between the 1’s with zeros 7510 = 1 001 0112





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Exercises Convert 110 0102 to decimal

Convert 100 110 101 1112 to decimal

Convert 27310 to binary by the division method

Convert 59710 to binary by the division method

Convert 532710 to binary by the subtraction method

Convert 1638410 to binary by the subtraction method





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Octal Numbering System Each octal numeral can be represented by 3 binary digits and the two number systems are readily converted from one to the other by substitution. So 778 is followed by 1008, in the similar sequence as 9910 is followed by 10010. 1008 is quite a bit smaller than 10010. 1008 = 6410. Has the advantages that a binary number sequence or readout can be displayed as an octal number which can then be readily converted by the engineer back to binary, eg for fault isolation. Instead of displaying 110 011 010 010 111 1102 on a readout, 6322768 can be displayed instead and it is far more easily written and remembered than the binary equivalent. Writing the binary number would increase the chances of transposing a digit when its copied, plus it is simply a large unwieldy number, difficult to write & difficult to memorise. 6322768 , by comparison is more simply remembered. A common method of interpreting data in aircraft is to memory inspect a computer memory location, and to interpret the data stored there. Of course the data is stored digitally. Some aircraft may have computer systems which convert the data into useable information, but others will simply only provide the digital data as it is stored. It is then the engineers task to interpret the digital data, eg On arriving back in the maintenance section the octal number 6322768 is easily converted back to binary and appropriate data interpreted, eg binary bits 8, 9 and 10 may all indicate undercarriage state (down and locked for example) to the computer, in this example these bits are 8 – low, 9 – low, 10 – high (this is counting the least significant bit {LSB} as “bit zero”). So in this example, binary bit 10 looks to be out of synch with binary bits 9 and 8, which could indicate a malfunctioning undercarriage microswitch or a broken signal wire. The aircraft maintenance manuals will provide the engineer with the memory inspection addresses (locations), and the information necessary to convert data, eg binary bit 8 is left main gear, binary bit 9 is right main gear, binary bit 10 is nose gear; a high or “1” equals undercarriage “not down and locked”, a low or “0” equals undercarriage down and locked. The octal numbering system has a base of 8 Numerals used 0 1 2 3 4 5 6 7 08 18 28 38 → 78 108 118 128 → 168 178 208 218 → 268 278 308 318 →768 778 1008 Each octal digit represents 3 binary digits

08 = 000 18 = 001 28 = 010 38 = 011 48 = 100 58 = 101 68 = 110 78 = 111


001 101 110 100 101 011 1 5 6 4 5 38

101 100 000 010 001 100 5 4 0 2 1 48

111 010 101 001 111 010 7 2 5 1 7 28

110 011 010 010 111 110 6 3 2 2 7 68




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Octal To Decimal Conversions One method is to convert the octal number to binary, and then convert the binary number to decimal as already explained by using the binary to decimal truth table. To convert straight across, use the octal truth table as explained To convert octal to decimal, 2 methods: •

First convert to binary, then convert binary to decimal as already explained

•

Use the octal truth table 85

84

83

82

81

80

32 76810

4 09610

51210

6410

810

110

+

5 x 81 +

1 x 80

0 x 6410 +

5 x 810 +

1x 110

Convert 20518 to decimal: 20518

= 2 x 83

+

= 2 x 51210 +

0 x 82

= 1 06510 Convert 362 4158 to decimal: 362 4158

=

3 x 85

+ 6 x 84

+ 2 x 83

+ 4 x 82

+ 1 x 81

+ 5 x 80

= 3 x 32 76810 + 6 x 4 09610 + 2 x 51210 + 4 x 6410 + 1 x 810 + 5 x 110 = 124 17310

Decimal To Octal Conversion There are two basic ways to do decimal-octal conversion. You may find one or the other easier to understand and use.

Method 1 – Decimal to Binary to Octal Convert the decimal number to binary as previously explained, then substitute each 3 binary bits, for an octal digit The advantage of this method is that if you learn how to convert everything to and from digital, you can use digital as the base system and need only remember how to convert each system to and from digital. Of the three systems we describe how to convert to and from binary, decimal is the only difficult method, Both octal and hexadecimal are easily converted to and from binary.





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Method 2 – Division Method The division method works on the same basis as the decimal to binary conversions already covered, but in this case we divide by “8”. All the remainders represent the octal number. To convert from decimal to octal you can convert decimal to binary, and then convert binary to octal, or Directly convert Decimal to Octal: very similar to converting decimal to binary by the division method Division Method

To convert 7 98510 to Octal 7985 ÷ 8 = 998 Rem 998 ÷ 8 = 124 Rem

To convert 18 36510 to Octal 18365 ÷ 8 2295

Rem 5

2295 ÷ 8 = 286

Rem 7

124 ÷ 8 = 15

Rem

286 ÷ 8 = 35

Rem 6

15 ÷ 8 = 1

Rem

35 ÷ 8 = 4

Rem 3

1÷ 8 = 0

Rem

4÷8= 0

Rem 4

798510 = 1 7 4 6 18

18 36510 = 4 3 6 7 58

Convert 110 0102 to octal

Convert 100 110 101 1112 to Octal

Convert 4218 to decimal using the octal truth table

Convert 11258 to decimal using the octal truth table

Convert 532710 to octal by the division method

Convert 16 38410 to octal by the division method





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Hexadecimal Numbering System The hexadecimal number system is referred to as base-16 and uses 16 unique symbols: 0 - 9 and A - F (the radix). This number system is useful because it can represent every byte (8bits of binary) as 2 symbols. Hex uses the first ten numbers of the decimal system: 0, 1, 2, 3, 4, 5, 6, 7, 8, and 9. The number 10 has two digits so in hexadecimal it is represented by the letter A. The number 11 by B, 12 by C, 13 by D, 14 by E and 15 by F. This gives sixteen single digit values: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F To go higher, to the decimal number 16, we must use two digits setting the first digit to 1 and increasing the second digit from 0 to F: 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 1A, 1B, 1C, 1D, 1E, 1F The decimal number 16 would equal 10 in hexadecimal. (1 X 16) + 0. Continuing in the series, 17(decimal) would equal 11(hexadecimal), 18(decimal) would equal 12(hexadecimal), and so on until 31(decimal), which would equal 1F(hexadecimal). To increment to the number 32, we must change the first digit to 2 and increase the second digit from F to 0: 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 2A ...and so on, and so on So FF16 is followed by 10016, in the similar sequence as 9910 is followed by 10010. 10016 is quite a bit larger than 10010. 10016 = 25610. The Hexadecimal numbering system has a base of 16 Numerals used 0 1 2 3 4 5 6 7 8 9 A B C D E F 016 116 216 316 → 916 A16 B16 → E16 F16 1016 1116 1216 → 1916 1A16 1B16 → 1E16 1F16 2016 2116 →9916 9A16 9B16 → 9E16 9F16 A016 A116 → FD16 FE16 FF16 10016 Each hexadecimal digit represents 4 binary digits

016 = 0000 116 = 0001 216 = 0010 316 = 0011 416 = 0100 516 = 0101 616 = 0110 716 = 0111

816 = 1000 916 = 1001 A16 = 1010 B16 = 1011 C16 = 1100 D16 = 1101 E16 = 1110 F16 = 1111


1101 1101 0010 1011 D D 2 B16 0010 1100 0000 1000 1100 2 C 0 8 C16 0011 1010 1010 0111 1010 3 A A 7 A16 0011 0011 0100 1011 1110 3 3 4 B E16




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Hexadecimal to Decimal Conversions One method is to convert the octal number to binary, and then convert the binary number to decimal as already explained by using the binary to decimal truth table. To convert straight across, use the hexadecimal truth table as explained The hexadecimal number 20, (2 X 16) + 0, equals 32 in decimal. The hexadecimal number 9C would equal (9 X 16) + 12 = 156 in decimal. To convert hexadecimal to decimal, 2 methods: •

First convert to binary, then convert binary to decimal as already explained

•

Use the hexadecimal truth table

165

164

1 048 57610 65 53610

163

162

161

160

4 09610

25610

1610

110

Convert B7F216 to decimal: B7F216

= 11 x 163

+

= 11 x 4 09610 +

7 x 162

+

7 x 25610 +

15 x 161

+

2 x 160

15 x 1610 +

2 x 110

= 47 09010 Convert 9B82A16 to decimal: 9B82A16

=

9 x 164

+ 11 x 163

+ 8 x 162

= 9 x 65 53610 + 11 x 4 09610 + 8 x 25610

+ 2 x 161

+ 10 x 160

+ 2 x 1610 + 10 x 110

= 636 97010 To convert a Decimal number to hexadecimal it is quite difficult to use the division method and divide by 16, but it will work Remainders higher than 9 must be represented by the appropriate letter

Division Method

To convert 7 98510 to

To convert 18 36510 to

7985 ÷ 16 499 Rem

18365 ÷ 16 1147 Rem D

499 ÷ 16 = 31

1147 ÷ 16 = 71

Rem

71 ÷ 16 = 4

Rem

4 ÷ 16 = 0

Rem

31 ÷ 16 = 1 1 ÷ 16 = 0

Rem Rem F Rem

798510 = 1 F 3 116 18 36510 = 4 7 B D16





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An easier method is to first convert the decimal number to binary Convert decimal to binary using either the division or subtraction method Substitute each 4 binary bits with a hexadecimal numeral 18 36510 =

0100

0111

=

4

7

=

0001

1111

=

1

F

1011 11012 B

D16

798510 0011 00012 3

116

Convert 0011 0010 2 to hexadecimal

Convert 1001 1010 11112 to hexadecimal

Convert 11116 to decimal using the Hexadecimal truth table

Convert 25516 to decimal using the Hexadecimal truth table

Convert 532710 to hexadecimal by the division method

Convert 16 38410 to binary then hexadecimal





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Summary The following text illustrates how to convert everything to binary and vice versa, using this common language (binary) conversions may be simpler Decimal10 to Binary2

• •

Decimal10 to Octal8

•

Division

Division Octal8 to Decimal10

Subtraction Binary2 to Decimal10

•

Octal truth table Octal8 to Binary2 to Octal8

• Binary truth table

•

Binary2 to Decimal10 to Binary2

Decimal10 to Hexadecimal16

•

• •

Division Hexadecimal16 to Decimal10

Binary truth table

Division or Subtraction Octal8 to Binary2 to Octal8

•

Hexadecimal truth table Hex16 to Binary2 to Hex16

•

3 binary bits = 1 Octal numeral

4 binary bits = 1 Hex numeral

•

3 binary bits = 1 Octal numeral Hex16 to Binary2 to Hex16

•

4 binary bits = 1 Hex numeral

Convert 48 87910 to hexadecimal

Convert DEAF16 to decimal

Convert 100 101 011 110 010 0112 to octal and hexadecimal

Convert 25516 to octal

Convert 86 43710 to octal

Convert 11 3248 to decimal





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Binary Coded Decimal (BCD) The decimal number system is easy to use because it is so familiar. The binary number system is less convenient to use because it is less familiar. It is difficult to quickly glance at a binary number and recognize its decimal equivalent. For example, the binary number 10100112 represents the decimal number 8310. It is difficult to tell immediately by looking at the number what its decimal value is. However, within a few minutes, using the procedures described earlier, you could readily calculate its decimal value. The amount of time it takes to convert or recognise a binary number quantity is a distinct disadvantage in working with this code despite the numerous hardware advantages. Engineers recognised this problem early and developed a special form of binary code that was more compatible with the decimal system. Because so many digital devices, instruments and equipment use decimal input and output, this special code has become very widely used and accepted. This special compromise code is known as binary coded decimal (BCD). The BCD code combines some of the characteristics of both the binary and decimal number systems. BINARY TO BCD CONVERSION Decimal-to-BCD conversion is simple and straightforward. However, binary-to-BCD conversion is not direct. An intermediate conversion to decimal must be performed first. For example, the binary number 1011.01 is converted into its BCD equivalent, i.e., First the binary number is converted to decimal. Then the decimal result is converted to BCD. 11.2510 = 0001 0001.0010 0101 To convert from BCD to binary, the previous operation is reversed. For example, the BCD number 1001 0110.0110 0010 0101 is converted into its binary equivalent. BCD TO BINARY CONVERSION To convert from BCD to binary, the previous operation is reversed. For example, the BCD number 1001 0110.0110 0010 0101 is converted into its binary equivalent. 1001 0110.0110 0010 0101 = 96.62510 First, the BCD number is converted to decimal. Then the decimal result is converted to binary. Because the intermediate decimal number contains both an integer and fraction, each number portion is converted as described under “Binary Number System.” The binary sum (integer plus fraction) 1100000.101 is equivalent to the BCD number 1001 0110.0110 0010 0101. Because the intermediate decimal number contains both an integer and fraction, each number portion is converted as described under “Binary Number System.” The binary sum (integer plus fraction) 1100000.101 is equivalent to the BCD number 1001 0110.0110 0010 0101.





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TOPIC 5.3 – DATA CONVERSION Data Conversion

Analogue-to-digital converter (ADC) and digital-to-analogue converter (DAC) are used to interface a computer to the analogue world so that a computer can monitor and control a physical variable. 1. Transducer – the physical variable is normally a non-electric quantity. A transducer is a device that converts that physical variable into an electrical variable. 2. ADC – The transducer’s electrical analogue output serves as the analogue input to the ADC. The ADC converts this analogue input into a digital output. This consists of a number of bits that represent the analogue value. Eg. The transducer may output an analogue voltage range of 800 to 1500 mV which the ADC might convert to 01010000 (80) to 10010110 (150). 3. Computer – The digital representation from the ADC is processed by the computer. It may perform calculations or other operations and then output a digital output to manipulate the physical variable. 4. DAC – The digital output from the computer is converted to a proportional analogue voltage or current. Eg. The computer output may output a digital range between 00000000 to 11111111, which the DAC converts to a voltage ranging from 0 to 10 volts. 5. Actuator – The analogue signal from the DAC is often connected to some device used to physically control or adjust the physical variable. An understanding of Op-amps used as comparators is required to understand the operation of A/D and D/A converters.





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Operational Amplifiers Operational Amplifiers are often used as comparators to compare the amplitude of one voltage with another. In this application, the op-amp is used in the open-loop configuration, with the input voltage on one input and a reference voltage on the other.

The term operational amplifier or "op-amp" refers to a class of high-gain DC coupled amplifiers with two inputs and a single output. The modern integrated circuit version is typified by the famous 741 op-amp. Some of the general characteristics of the IC version are: High gain, on the order of a million High input impedance, low output impedance (Vo = Av x Vin ie. Voltage out = Gain x Voltage in) Used with split supply, usually +/- 15V Used with feedback, with gain determined by the feedback network.





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Zero Level Detection One application of an op-amp used as a comparator is to determine when an input voltage exceeds a certain level. Note in the illustration that the inverting input is grounded to produce a zero level and that the input signal is applied to the non-inverting input.

Because of the high open-loop voltage gain, a very small difference between the two inputs drives the op-amp into saturation, causing the output voltage to go to its limit. For example, consider an op-amp having a gain of 100,000. A voltage difference of only 0.25 mV between the inputs could produce an output voltage of 25 Volts if the op-amp were capable. However, since most op-amps have a maximum output voltage limitation of +/- 15 V because of their DC supply voltages, the device would be driven into saturation. The waveshape illustration shows the result of a sinewave input voltage applied to the noninverting input of the zero-level detector. When the sinewave is negative, the op-amp output is at its maximum negative level. When the sinewave input crosses zero (going positive), the amplifier is driven to its opposite state and the output goes to its maximum positive level. The zero-level detector can be used as a squaring circuit to produce a square wave from a sinewave.

Non-zero Level Detection The zero-level detector can be modified to detect voltages other than zero by connecting a fixed reference voltage to the inverting input as shown in illustration (a) using a battery. A more practical arrangement is shown in figure (b) using a voltage divider to set the reference voltage. A zener diode can also be used to set the reference voltage.

As long as the input voltage (Vin) exceeds the reference voltage (Vref), the output goes to its maximum positive voltage as shown in the waveform illustration.





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Comparator A more general comparison circuit is shown.

This circuit is a true comparator, in that it corrrectly indicates a voltage comparison between a reference voltage (Vref) and an unknown input voltage (Vin) at the other. As with the original zero crossing detector above, the two inputs may be swapped according to the desired sense of the output. The resistor and Zener diode in the output circuit convert the full-range output swing to digital levels, if that is desired. However, these components are not required for the basic voltage comparison. Because this circuit effectively compares the two input voltages and produces a corresponding output, it is known as a comparator. Comparators find a wide range of applications in practical, commercial circuits.





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Non-inverting Amplifier An op-amp connected in a closed-loop configuration as a non-inverting amplifier with a controlled amount of voltage gain is shown in figure.

The input signal is applied to the non-inverting input. The output is applied back to the inverting input through the feedback circuit (closed loop) formed by Ri and Rf. This creates negative feedback as follows. Ri and Rf form a voltage divider circuit which reduces Vout and connects the reduced voltage to the inverting input. The feedback voltage is expressed as:

The closed-loop gain of the non-inverting amplifier can be calculated with:

Therefore the closed-loop gain can be set by selecting values of Rf and Ri.





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Inverting Amplifier

For an ideal op-amp, the inverting amplifier gain is given simply by:

For equal resistors, it has a gain of -1, and is used in digital circuits as an inverting buffer. An opamp inverting amplifier with a gain of one serves as an inverting buffer.

Digital-to-Analogue Conversion One common requirement in electronics is to convert signals back and forth between analogue and digital forms. Most such conversions are ultimately based on a digital-to-analogue converter circuit. Therefore, it is worth exploring just how we can convert a digital number that represents a voltage value into an actual analogue voltage.





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Binary Weighted Resistor D/A Converter The illustrated circuit is a binary weighted resistor D/A converter. It assumes a 4-bit binary number in Binary-Coded Decimal (BCD) format, using +5 volts as a logic 1 and 0 volts as a logic 0. It will convert the applied BCD number to a matching (inverted) output voltage. The digits 1, 2, 4, and 8 refer to the relative weights assigned to each input. Thus, 1 is the Least Significant Bit (LSB) of the input binary number, and 8 is the Most Significant Bit (MSB). If the input voltages are accurately 0 and +5 volts, then the "1" input will cause an output voltage of -5 × (4k/20k) = -5 × (1/5) = -1 volt whenever it is a logic 1. Similarly, the "2," "4," and "8" inputs will control output voltages of -2, -4, and -8 volts, respectively. As a result, the output voltage will take on one of 10 specific voltages, in accordance with the input BCD code. The operational amplifier is employed as a summing amplifier, which produces the weighted sum of these input voltages. It may be recalled that the summing amplifier multiplies each input voltage by the ratio of the feedback resistor Rf to the corresponding input resistor Rin. In this circuit, Rf = 4 K and the input resistors range from 2.5 K to 20 K. Eg. The ‘1’ input has Rin = 20 K, and so the summing amplifier attenuates the signal so the output is 4/20th ( 1/5th ) that of the input (and inverted). The analogue output voltage represents the weighted sum of the digital inputs (BCD) Illustration shows the conversion for a binary-weighted resistor D/A Converter.





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R/2R Ladder DAC The DAC circuits we have looked at thus far use binary-weighted resistors to produce the proper weighting of each bit. While this method works in theory, it has some practical limitations. The biggest problem is the large difference in resistor values between the LSB and the MSB especially in high-resolution DACs (ie. Many bits). For example, if the MSB resistor is 1 K in a 12-bit binary weighted resistor DAC, the LSB resistor will be over 2 M. It is very difficult to produce resistance values over a wide range that maintain an accurate ratio especially with variations in temperature.

The circuit above performs D to A conversion a little differently. Typically the inputs are driven by CMOS gates, which have low but equal resistance for both logic 0 and logic 1. Also, if we use the same logic levels, CMOS gates really do provide +5 and 0 volts for their logic levels. Consider that for each input, if it is NOT a HI (+ 5 V) , then it is a LO (0 V). The input circuit is a remarkable design, known as an R-2R ladder network. It has several advantages over the binary weighted resistor DAC. Only two resistance values are used anywhere in the entire circuit. This means that only two values of precision resistance are needed, in a resistance ratio of 2:1. Therefore, resistance variations due to temperature changes have little effect on the accuracy. This requirement is easy to meet, and not especially expensive. We can use the very standard values of 10k and 20k for our resistors. The circuit is indefinitely extensible for binary numbers. Thus, if we use binary inputs instead of BCD, we can simply double the length of the ladder network for an 8-bit number (0 to 255) or double it again for a 16bit number (0 to 65535). We only need to add two resistors for each additional binary input. The input resistance seen by each digital input is the same as for every other input. The actual impedance seen by each digital source gate is 3R. The circuit lends itself to a non-inverting circuit configuration. Therefore we need not be concerned about intermediate inverters along the way. However, an inverting version can easily be configured if that is appropriate.





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One detail about this circuit: Even if the input ladder is extended, the output will remain within the same output voltage limits. Additional input bits will simply allow the output to be subdivided into smaller increments for finer resolution. This is equivalent to adding inputs with ever-larger resistance values (doubling the resistance value for each bit), but still using the same two resistance values in the extended ladder.

The basic theory of the R-2R ladder network is actually quite simple. Current flowing through any input resistor (2R) encounters two possible paths at the far end. The effective resistances of both paths are the same (also 2R), so the incoming current splits equally along both paths. The half-current that flows back towards lower orders of magnitude does not reach the op amp, and therefore has no effect on the output voltage. The half that takes the path towards the op amp along the ladder can affect the output. The most significant bit (marked "8" in the figure) sends half of its current toward the op amp, so that half of the input current flows through that final 2R resistance and generates a voltage drop across it. This voltage drop (from bit "8" only) will be one-third of the logic 1 voltage level, or 5/3 = 1.667 volts. This is amplified by the op amp, as controlled by the feedback and input resistors connected to the "-" input. For the components shown, this gain will be 3 (refer to non-inverting amplifiers – Gain). The gain for non-inverting amplifiers can be calculated by:

With a gain of 3, the amplifier output voltage for the "8" input will be 5/3 × 3 = 5 volts. The current from the "4" input will split in half in the same way. Then, the half going towards the op amp will encounter the junction from the "8" input. Again, this current "sees" two equalresistance paths of 2R each, so it will split in half again. Thus, only a quarter of the current from the "4" will reach the op amp. Similarly, only 1/8 of the current from the "2" input will reach the op amp and be counted. This continues backwards for as many inputs as there are on the R-2R ladder structure. The maximum output voltage from this circuit will be one step of the least significant bit below 10 volts. Thus, an 8-bit ladder can produce output voltages up to 9.961 volts (255/256 × 10 volts). This is fine for many applications. If you have an application that requires a 0-9 volt output from a BCD input, you can easily scale the output upwards using an amplifier with a gain of 1.6 (8/5).





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We will not perform a detailed analysis of this circuit here, but it can be shown that, in this instance, the value of Vout is given by the expression:

where B is the value of the binary input, which can range from 0000 (0) to 1111 (15), and the denominator is the value of the MSB (8 in this instance with a 4-bit input). With a 3-bit input, the denominator (MSB value) would be 4.

Analogue-to-Digital Conversion A/D conversion is a common interfacing process often used when a linear analogue system must provide inputs to a digital system. Many methods for A/D conversion are available. We will cover the basic operation of 2 A/D converter types: •

Flash or simultaneous

•

Digital-Ramp or Counter-type

The A/D conversion process is generally more complex and time-consuming than the D/A process, and many different methods have been developed and used. It may never be necessary to design or construct ADCs (they are available as complete packaged units). However, the techniques that are used provide an insight into what factors determine an ADCs performance.





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Flash A/D Converter To convert a digital code to an analogue voltage, we only had to find a way to effectively assign an appropriate voltage to each bit, and then combine them. But is there some equally easy way of finding the digital code that corresponds to a given analogue voltage?

Consider the very simple requirement to determine whether an analogue voltage was closest to 0, 1, 2, or 3 volts. The result would be stored as a two-bit binary number. The first step in making this determination might be a set of three comparators, connected as shown. As the analogue voltage increases, the comparators will, one by one from the bottom up, change state from false to true. Of course, additional digital circuitry will be required to encode these signals into the corresponding digital number. But this circuit forms the sensing array that will determine directly which code will be closest to the actual analogue voltage. This approach will work, and can be expanded to any number of steps for finer resolution of the analogue voltage. However, as you have probably already perceived, there is a problem with this approach, in that the number of comparators required increases exponentially with the number of binary bits used to store the code. Thus, using this approach to convert a 0 to 9-volt range to a BCD number will require nine comparators. A 4-bit binary number, counting from 0 to 15, requires 15 comparators. And a typical 8-bit circuit will require 255 comparators! Clearly this approach becomes rapidly too expensive for ordinary use, although it is practical if very high speed is required.





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Flash A/D Converter - Encoder Due to the nature of the sequential comparator output states (each comparator saturating "high" in sequence from lowest to highest), the same "highest-order-input selection" effect may be realised through a set of Exclusive-OR gates, allowing the use of a simpler, non-priority encoder. The encoder circuit itself can be made from a matrix of diodes, demonstrating just how simply this converter design may be constructed. Not only is the flash converter the simplest in terms of operational theory, but it is the most efficient of the ADC technologies in terms of speed, being limited only in comparator and gate propagation delays. Unfortunately, it is the most componentintensive for any given number of output bits.

An additional advantage of the flash converter, often overlooked, is the ability for it to produce a non-linear output. With equal-value resistors in the reference voltage divider network, each successive binary count represents the same amount of analog signal increase, providing a proportional response. For special applications, however, the resistor values in the divider network may be made non-equal. This gives the ADC a custom, nonlinear response to the analog input signal. No other ADC design is able to grant this signal-conditioning behavior with just a few component value changes.





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Digital-Ramp ADC A slower but much less expensive approach involves the use of a D to A converter and a single comparator. One of the simplest versions of the general ADC uses a binary counter as the register and allows the clock to increment the counter one step at a time until Vax ≥ Va. It is called a digital-ramp ADC because the waveform at Vax is a step-by-step ramp. It may also be referred to as a counter-type ADC. It contains a counter, a DAC, an analogue comparator, and a control AND gate. The comparator output serves as the active-LO EOC signal.

If we assume that Va (analogue voltage) is positive , the operations proceeds as follows:  A START pulse is applied to reset the counter to zero. The HI at START also inhibits clock pulses from passing through the AND gate into the counter.  With all 0’s at its input, the DAC output will be Vax = 0 Volts.  Since Va > Vax, the comparator output (EOC) will be HI.  When START returns LO, the AND gate is enabled and the clock pulses now advance through to the counter.  As the counter advances, the DAC output (Vax) increases one step at a time.  This continues until Vax exceeds Va (typically by 10 to 100 microvolts). At this point, the comparator changes state and its output goes LO. This will inhibit the flow of pulses into the counter and thus it stops counting.  The conversion process is now complete as signalled by the HI to LO transition at EOC, and the contents of the counter are the digital representation of Va.  The counter will hold the digital value until the next START pulse initiates a new conversion.





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TOPIC 5.4 – DATA BUSES Electricity has many uses. One common use is to perform work. In electricity delivery grids from power stations, power it transmitted over heavily constructed thick power lines designed to carry very high voltages and a large electrical current. As the power is stepped down, so is the diameter of the wiring necessary to conduct it efficiently. Different applications require differing amounts of power, for example a navigation light runs off 28 VDC, and so does an aircraft starter motor. The navigation light only draws a very small current though, as is evident by the narrow gauge wiring providing the power to it. The starter motor though draws a very high current so a thick cable is necessary for the motor to operate efficiently and generate the torque required to turn over an aircraft engine. Both these aircraft applications though use electricity to perform work, hence there is significant current flow, necessitating appropriately sized wiring to carry the current required. An entirely different use for electricity is to transmit signals. Electricity is the ideal means to transmit information because it travels at the speed of light. Wiring utilised to transmit data carries only a small negligible current, just sufficient to switch a transistor ON or OFF, or to carry an audio signal which is then amplified at its destination. This lesson deals entirely with transmission of data. Ideally no current flows on a digital data line, although in reality there is a small flow of current sufficient to at least fwd and reverse bias semiconductor P&N junctions. But data bus lines are typically very small gauge, and the electrical signal transmitted over them is typically no higher than 5 volts DC and would look similar to an AC sine wave, although without any uniformity or sequence. The information transmitted is actually all the ones and zero’s which represent data encoded as a digital signal. The data is sent in a regulated and uniform sequence between components, where computer processors at either end decode and utilise the date to produce the desired outputs, whether it be to display present latitude and longitude on a horizontal indicator, or to drive a servo motor to regulate fuel flow to an engine.



Topic 5.4: Data Buses Page 1 of 30


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Digital Data Transfer The signal below represents an ideal waveform, below is an example of the more likely waveform present on the data line. Both represent 1010 0110. Remember, the voltage produced is as a result of transistor switching ON and OFF and transistors are not perfect in respect to having instantaneous switching of states from OFF to fully saturated. They are continually forward and reverse biased, so the waveshape is in reality more like a distorted AC sine wave. Although the waveshapes are not perfect, they do function perfectly, and any computer is testament to how well the digital data transfer works.

In electronic digital systems data in its binary form is represented by the presence or not of a voltage for each bit at the inputs and outputs of the various circuits. Typically binary 0 is represented by 0 volts, and binary 1 is represented by 5 volts. In practical systems any voltage between 0 and 0.8 volts (not sufficient to saturate a transistor) represents binary 0 and any voltage between 2 and 5 volts represents a binary 1.

The clock pulse represented on the diagram is basically a representation of the operating speed of the data bus, and the transmitter and receiver are synchronised by the same clock pulse. When the transmitter is outputting a high, the receiver detects it and clocks it through as a 1 to processing circuitry. When data is next sampled by the receiver the transmitter is outputting a low, and a 0 is clocked through to the processing circuitry. In digital, quantities are represented by voltages which have a wide tolerance (eg: 2-5 volts for a 1), whereas in analogue, voltages must be exact – any deviation causes errors.





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Serial Data Transfer In digital computers, enormous amounts of data moves between parts of the system. The two basic ways of doing this are by “parallel data transfer” and “serial data transfer”. With serial transfer, each bit of data is transferred from a store (or memory location, illustrated is two bytes of data – 16 bits) in sequence over the same line. The data is triggered by clock pulses as explained on the previous slide and the transmitter and receiver are synchronised, with reference to the same clock pulse. When the data arrives at the receiver it is sequentially stored in memory (2 bytes worth in this case) before being transferred to processing circuitry in the receiving component. The serial bus is one on which the data is transmitted sequentially, one word following another word. It is commonly used for long distance transmissions. Advantages of Serial data flow: less hardware, therefore less weight and space for an installation compared to Parallel data transfer systems. Serial data transfer is typical of data-bus communications. Multiplexing is a typical method of speeding up the data transfer capacity of a serial data bus.

Parallel Data Transfer With parallel transfer, each bit is taken from a separate circuit (eg – processing or calculating circuit) and is transmitted over a separate line.

Advantages of parallel data transfer, much faster. In the example on the slide, it would be 16 times faster. When considering the time taken to download data from the internet over the serial connection, imagine how quickly everything would run if there was a parallel connection. The downside of parallel is of course you need lots more hardware which takes up space and increases weight, which are two things we don’t want to do in an aircraft. Serial data transfer is typical of data-bus communications, whereas once a signal is inside a computer it is typically processed in parallel. Parallel bus typically interconnects the internal devices of a computer and has enough wires to transmit all bits of the word simultaneously. An eight bit parallel bus would be eight times faster than the serial bus, and a 64 bit parallel bus would be 64 times faster than its equivalent serial bus.





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Multiplexing Multiplexing – combining two or more info channels onto a common transmission medium. On aircraft, multiplexing greatly decreases the number of wires carrying separate signals. Using digital ‘time division” technique many different signals can be carried by one conductor. Benefits include a big reduction in weight of wire bundles & improved circuit reliability. The basic principle of multiplexing is where two rotary switches are synchronised in their switching as they rotate around a series of contacts. The synchronised rotating contacts connect matching input & output lines in sequence, and data is transmitted over the common transmission line.

In reality multiplexing is usually done by logic gates responding in sequence to clock pulse signals. At the multiplexing end the signal on each input line is sampled and passed to the common transmission line when the inputs AND gate is clocked ON. The sequenced gate outputs are serially transmitted to the demultiplexer where the inverse happens. As each AND gate is clocked on it passes the signal that is on the transmission line at that time. This has the effect of transmitting 8 separate inputs through to 8 separate outputs over the same transmission line. In aircraft, analogue signals may be multiplexed, but they must first be converted to digital, transmitted over the multiplexer network, then converted back to analogue form once demultiplexed. In an aircraft the sequencing controller is replaced by a Bus Controller, and the bus controller typically receives all the inputs, and distributes outputs and processed data (after processing data from several inputs) to systems requiring the information, eg Digitised data to be displayed on a multifunction display, or calculated air density data for transmission to a thrust computer.





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Aircraft Multiplex System In the 1950s and 1960s, aviation electronics, referred to as avionics, were simple stand-alone systems. The navigation, communications, flight controls, and displays consisted of analog systems. Often these systems were composed of multiple boxes, or subsystems, connected to form a single system. Various boxes within a system were connected with point-to-point (analogue) wiring. The signals mainly consisted of analog voltages, synchro-resolver signals, and switch contacts. The location of these boxes within the aircraft was a function of operator need, available space, and the aircraft weight and balance constraints.

As more and more systems were added, the cockpits became more crowded, the wiring more complex, and the overall weight of the aircraft increased. By the late 1960s and early 1970s, it became necessary to share information between the various systems to reduce the number of black boxes required by each system. A single sensor, for example that provided heading and rate information, could provide that data to the navigation system, the weapons system, the flight control system, and pilots display system. However, the avionics technology was still basically analog, and while sharing sensors did produce a reduction in the overall number of black boxes, the connecting signals became a “rat's nest” of wires and connectors. Moreover, functions or systems that were added later became an integration nightmare, as additional connections of a particular signal could have potential system impacts. Additionally, as the system used point-to-point wiring, the system that was the source of the signal typically had to be modified to provide the additional hardware to output to the newly added subsystem (additional amplifiers, or output multipliers boxes). Because a single parameter may be required by several boxes or systems, it was necessary to incorporate output multiplier boxes (OMB’s) where a single signal (or range of signals, eg ADC OMB) would be fed into analogue multipliers, so the signal could be replicated many times for output to the associated systems requiring the information, eg attitude for display, autopilot, radar transmitter stabilisation, etc. Output multipliers were large heavy boxes and added to aircraft weight & space problems, as well as adding complexity to systems operation.





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The analogue signals all require dedicated wiring to pass the information from one box to another. In later day computerised aircraft it would likely be impossible to design or construct an aircraft with analogue wiring, because too much information to be handled by analogue wiring is typically transmitted for routine operation of the avionic systems. The aircraft would be a flying wiring loom.

By the late 1970s, with the advent of digital technology, digital computers had made their way into avionics systems and subsystems. They offered increased computational capability and easy growth, compared to their analog predecessors. However, the data signals, inputs and outputs from the sending and receiving systems were still mainly analog in nature. This led to the configuration of a small number of centralized computers (typically only one or two) being interfaced to other systems and subsystems via complex and expensive analog-todigital and digital-to-analog converters. As time and technology progressed, the avionics systems became more digital. And with the advent of the microprocessor, things really took off. A benefit of this digital application was the reduction in the number of analog signals, and hence the need for their conversion. An additional side benefit was that digital data could be transferred bi-directionally, wherein analog data was transferred unidirectional. Multiplexing is a technique that minimises the amount of wiring required to transmit information, or commands throughout an aircraft. It is not unique to aircraft, motor vehicles have also been using multiplexing for many years. By multiplexing, a bus controller manages all communications over the multiplexing bus. The bus controller works similar to a base station radio operator. In the illustration, bus controller is performed by the FMC. The mission computers control all data transmitted over the multiplexer busses. The incorporation of a fully integrated digital avionics system requires a digital data bus to provide a two way interface between various navigation sensors, computers and indicators.





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Serial rather than parallel transmission of the data was used to reduce the number of interconnections (wires) within the aircraft and the receiver/driver circuitry required with the black boxes. The interface between each computer or external device is accomplished via the digital data bus. The bus is made up of a twisted pair of wires which are shielded and jacketed. The shielding is to provide spike protection and eliminate EMF induced errors and virtually guarantees accurate transmissions from each transmitter to its receiver. The wires are twisted so the magnetic fields induced by the currents flowing through them will cancel each other out eliminating electro-magnetic interference. Data may travel one way or in two directions, depending on the system design. Transmission of data within micro-computers and external transmissions between other components is accomplished with 8, 16, 32 or 64 bit digital words.

Regardless of which system is employed, only one data word will be transmitted on the data bus at any one time, it is not a free for all, as then the data bus would simply be a jumble of one’s & zero’s & would be meaningless. All communication is controlled by either bus controllers or by timing regimes, like a radio communications system. If several stations transmit over the same frequency simultaneously – non can be understood – each transmission must be timed to transmit one at a time, then all radio traffic can be understood.





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Data Bus Connectors Breakouts to specific equipment is normally accomplished at “Burndy” block type modules The multiplexer bus functions like an arterial highway, and is not designed to connect to any specific components. The highway is laid, and a bus controller is connected to manage all data transmitted over the highway. All the peripheral components are connected to the highway by breakouts (couplers) and perform similar to telephone extensions connected to an exchange, or computers connected to a LAN.

Data Bus Systems: Bus Controller The bus controller is responsible for directing the flow of data on the data bus. While several terminals may be capable of performing as the bus controller, only one bus controller may be active at any one time. The bus controller is the only one allowed to issue commands on the data bus. Commands may be for transfer of data, or control & management of the bus.

Bus Controllers manage this by sending out commands to peripheral systems requesting data which is stored in the memory of the peripheral components, eg altitude from the ADC. The ADC responds to the digital command and transmits binary data representing aircraft altitude. The MC then updates the EFIS altitude readout by transmitting the digitised altitude value to the EFIS, where the new altitude value is displayed. Data transmission is strictly controlled by the bus controllers. There are typically only a few types of word transmitted over digital data busses, and all communications on MIL-STD-1553 type data busses are initiated by the Bus Controller. The commands may be for the transfer of data or the control and management of the bus (referred to as mode commands. The Bus controller commands a remote terminal to transmit data to it. The remote terminal responds with the information requested. The bus controller may command a remote terminal to receive data. It sends the data, eg digitised information for display by a multifunction display, and the remote terminal responds when the data transfer is complete to confirm that it received the information and that no errors were detected in the transmission. In this way





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enormous amounts of information are transferred along the data bus all under the control of the bus controller. Typically, the bus controller is a function that is contained within some other computer, such as a flight management computer, display processor, or a fire control computer (military).

MIL–STD–1553 Data Bus MIL-STD-1553 - military standard defining electrical & protocol characteristics for a data bus. The Advent of the digital Data Bus alone was still not enough. A data transmission medium, which would allow all systems and subsystems to share a single and common set of wires, was needed. By sharing the use of this interconnect, the various subsystems could send data between themselves and to other systems and subsystems, one at a time, and in a defined sequence, hence a data bus. MIL-STD-1553B defines the term Time Division Multiplexing (TDM) as “the transmission of information from several signal sources through one communications system with different signal samples staggered in time to form a composite pulse train”. This means that data can be transferred between multiple avionics units over a single transmission media, with the communications between the different avionics boxes taking place at different moments in time, hence time division. MIL-STD-1553 (USAF) was released in August of 1973. The primary user of the initial standard was the F-16. Further changes and improvements were made and a tri-service version, MILSTD-1553A was released in 1975. The first users of the “A” version of the standard was the Air Force's F-16 and the Army's new attack helicopter, the AH-64A Apache. With some “real world” experience, it was soon realised that further definitions and additional capabilities were needed.





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The SAE spent three years of concentrated effort to produce 1553B, which was released in 1978. At that point, the government decided to “freeze” the standard at the “B” level so as to allow component manufacturers to develop products and to allow industry to gain some additional “real world” experience before determining what the next set of changes were to be made.

Three distinct word types are defined by the standard. These are: •

Command words.

•

Data words.

•

Status words.

Each word type has a unique format, yet all three maintain a common structure. Each word is twenty bits in length. The first three bits are used as a synchronisation field, thereby allowing all device clocks to re-sync at the beginning of each new word. The next sixteen bits are the information field and are different between the three word types. The last bit is the parity bit. Parity is based on odd parity for the single word. The encoder automatically calculates parity. Odd Parity, that is there is always an odd number of 1’s in a word. Command words contain a terminal address which indicates to the remote terminals which component the command is addressed to. T/R bit signifies whether the remote terminal is to prepare to receive data, or is to transmit data. Sub-address mode indicates the memory location the remote terminal is to either store transmitted data in, or is to transmit data from.





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Word count indicates how many data words are about to be sent to the RT or how many words the RT must transmit back to the BC. If the sub-address area contains all zero’s this indicates the command word is a mode change, and then the word count block contains data indicating which mode the RT is to switch to, eg switch an Inertial Navigation Unit from alignment mode to navigation mode, or command an ADC to perform a Built-in-Test.

Data words contain purely data, and are always preceded by a command or status word to effectively label what data is contained in the data words Status words contain the terminal address the status word is sent from so the BC knows who it’s talking to. The remainder of the word basically tells the BC that the data transfer was completed successfully and that the remote terminal is serviceable and operating correctly. Bit encoding for all words is based on bi-phase Manchester II format. The Manchester waveform is illustrated below. A transition of the signal occurs at the centre of the bit time. A logic “0” is a signal that transitions from a negative level to a positive level. A logic “1” is a signal that transitions from a positive level to a negative level.





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It is important to note that the voltage levels on the bus are not the signaling media, and that it is strictly the timing and polarity of the zero crossings that convey information on the bus. That is the ramps up or down indicate a 0 or a 1, not the magnitude of voltage. For this reason the 1553 bus is extremely forgiving of conditions that cause the voltage levels on the bus to vary

MIL–STD–1553 Data Transfer The primary purpose of the data bus is to provide a common media for the exchange of data between systems. The exchange of data is based on message transmissions. The standard defines ten types of message transmission formats. All of these formats are based on the three word types just defined. For the purpose of explaining how the data transfers are achieved, only 3 of the message formats are shown on the slide and will be explained in some detail. The remaining 7 message formats are: 3 Mode change transmissions •

BC Commands mode change of RT, without any data being transferred

•

BC transmits Mode Change command of RT and transmits data to the RT

•

BC transmits Mode Change command of RT and requests data be transmitted from RT to BC

4 Broadcast message transmissions •

BC to RT’s, BC transmitting data

•

BC commands RT’s to transmit data to other RT’s

•

BC transmits Mode Change command to RT’s without any data being transferred

•

BC transmits Mode Change command to RT’s and transmits data to RT’s

There are two message format groups, the information transfer formats and the broadcast information transfer formats. The information transfer formats are based on the command/response philosophy in that all error free transmissions received by a remote terminal are followed by the transmission of a status word from the terminal to the bus controller. This handshaking principle validates the receipt of the message by the remote terminal. Each of the message formats is summarized in the sections, which follow.

Bus Controller (BC) transmitting data to Remote terminal (RT) The bus controller to remote terminal (BC-RT) message is referred to as the receive command since the remote terminal is going to receive data. The bus controller outputs a command word to the terminal defining the Sub address of the data and the number of data words it is sending (sub address informs the receiver of the memory location where the data is to be stored). Immediately (no transmission gap), the number of data words (up to 32) specified in the





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command word is sent. The remote terminal upon validating the command word & all data words issues a status word indicating the message was received & was valid.

Remote terminal (RT) transmitting data to Bus Controller (BC) The remote terminal to bus controller (RT-BC) message is referred to as a transmit command. The bus controller issues only a transmit command word to the remote terminal. The terminal, on validating the command word, transmits its status word followed by the number of data words requested by the command word.

Remote Terminal (RT) transmitting data to Remote Terminal (RT) The remote terminal to remote terminal (RT-RT) command allows a terminal (the data source) to transfer data directly to another terminal (the data sink) without going through the bus controller. However, the bus controller may also collect the data & use it. The bus controller issues a command word to the receiving terminal immediately followed by a command word to the transmitting terminal. The receiving terminal is expecting data, but instead of data after the command word it sees a command sync (the second command word). The receiving terminal ignores this word and waits for a word with a data sync. The transmitting terminal ignored the first command word (it did not contain the appropriate terminal address). The second word was addressed to it, so it processes the command as a RTBC command by transmitting its status word followed by the required data words. The receiving terminal, having ignored the second command word, again sees a command (status) sync on the next word and waits. The next word (the first data word sent) now has a data sync and the receiving remote terminal starts collecting data. After receipt of all of the data words (and validating), the terminal transmits its status word.

MIL–STD–1553 Specifications An example of the standards specified by MIL-STD-1553.

The 1553 bus is bidirectional, in that data flows in both directions (not simultaneously) from BC to RT and from RT to BC. This means the bus must be managed by a bus controller coordinating





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all the data traffic. ARINC 429 is simplex operation. A component transmits data to up to 20 terminals, but data does not flow in reverse. As time and technology have progressed avionics systems became more digital. With the advent of the microprocessor, things really took off. Small analogue sensors could incorporate a microprocessor, thus providing a digital output and negating the requirement for A-to-D & D-to-A converters. 1553 systems incorporated many A-to-D & D-to-A converters, increasing the cost of installations. With avionics systems now typically producing outputs in digital format the 1553 standard of installation has fallen behind contemporary digital avionic system installations. So although MIL-STD-1553 was a pioneer in digital data bus development, ARINC standard installations are now more typically incorporated in modern commercial aircraft.

MIL-STD-1773 MIL-STD-1773 contains the requirements for utilising a fibre optic "cabling" system as a transmission medium for the MIL-STD-1553B bus protocol. As such, the standard repeats MILSTD-1553 nearly word-for-word. The standard does not specify power levels, noise levels, spectral characteristics, optical wavelength, electrical/optical isolation or means of distributing optical power. These must be contained in separate specifications for each intended use. Data encoding and word format are identical to MIL-STD-1553, with the exception that pulses are defined as transitions between 0 (off) and 1 (on) rather than between + and - voltage transitions since light cannot have a negative value. Since the standard applies to cabling only, the bus operates at the same speed as it would utilising wire. Additionally, data error rate requirements are unchanged. Different environmental considerations must be given to fibre optic systems. Altitude, humidity, temperature, and age affects fibre optics differently than wire conductors. Power is divided evenly at junctions which branch and connectors have losses just as wire connectors do.

History of ARINC Aeronautical Radio, Incorporated (ARINC) is a major company that develops and operates systems and services to ensure the efficiency, operation, and performance of the aviation and travel industries. It was organised in 1929 by four major airlines to provide a single licensee and coordinator of radio communications outside the government. Only airlines and aviation-related companies can be shareholders, although all airlines and aircraft can use ARINC’s services. ARINC has provided leadership in developing specifications and standards for avionics equipment, and one of these specifications is the focus of this lesson. Industry-wide committees prepare the specifications and standards. ARINC Specification 429 was developed and is maintained by the Airlines Electronic Engineering Committee (AEEC) comprising members that represent airlines, government, and ARINC. The General Aviation Manufacturers Association (GAMA) in Washington, D.C. also maintains a specification document with ARINC 429 labels: “ARINC 429 General Aviation Subset”.

What is ARINC 429? ARINC 429 is a specification, which defines how avionics equipment and systems should communicate with each other. They are interconnected by wires in twisted pairs. The specification defines the electrical and data characteristics and protocols, which are used. ARINC 429 employs a unidirectional data bus. Messages are transmitted at a bit rate of either 12.5 or 100 kilobits per second to other system elements, which are monitoring the bus messages. Transmission and reception is on separate ports so that many wires may be needed on aircraft, which use a large number of avionics systems.





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ARINC 429 Usage ARINC 429 has been installed on most commercial transport aircraft including; Airbus A310/A320/A330/A340; Bell Helicopters; Boeing 727, 737, 747, 757, & 767; & McDonnell Douglas MD-11. Boeing is installing a newer system specified as ARINC 629 on the 777. Some aircraft are using alternate systems in an attempt to reduce the weight of wire needed & to exchange data at a higher rate than is possible with ARINC 429. The unidirectional ARINC 429 system provides high reliability at the cost of wire weight & limited data rates.

Development of ARINC 429 A number of digital transmission system building blocks were available prior to 1984. Many protocols predate ARINC 429 such as ARINC 561, 582, 573, 575 and 419. The variability of standards doesn't matter where a single user is involved, but is important when equipment from different suppliers must interact with each other. Standardisation is beneficial not only to the aircraft integrator, but to the equipment supplier who can have greater assurance of product acceptability so long as it is "on spec". ARINC 429 is the most widely applied Digital Data Transmission specification for modern transport aircraft. The existence of ARINC 429 means avionic equipment manufacturers need not have to make components specific to certain aircraft or manufacturer types. The alternative with all air carriers utilising different data busses would mean manufactures of say a laser ring gyro would then need to manufacture variants to comply with each carriers data bus specifications. By sticking to a standard, manufacturers can produce standard products which can be incorporated into any aircraft, thus keeping manufacturing costs down resulting in massive savings to the airlines purchasing aircraft and spare parts.

ARINC 429 Specifications This lesson makes no attempt to provide sufficient information about ARINC 429 for you to comprehensively understand the details of the specifications, as long as you understand the concept and some of the major characteristics of ARINC 429, you will have achieved the lesson objectives. Some of the major characteristics include: •

data bus uses two signal wires

•

Word size – 32 bits (1553 was 20 counting polarity and synch)

•

Bit Encoding – Bipolar return to zero (1553 was Manchester II Bi-Phase, triggered by +ve & -ve going pulses)

•

Simplex Data Bus (1553 was bidirectional data bus)

A simplex bus is one on which there is only one transmitter but multiple receivers (up to a maximum of 20 in the case of 429). There are no Bus Controllers as found in 1553 buses. Since each bus is unidirectional, a system needs to have its own transmit bus if it is required to respond or to send messages. ARINC 429 specifies that bi-directional data flow on a pair of wires is not permitted. Requirements for minimum weight and maximum flexibility drove 1553 to





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operate at 1Meg on a bidirectional bus, certification requirements drove ARINC 429 to operate at either 12 to 14.5 or 100 k bit on a simplex bus.

ARINC 429 Schematic Diagram

There is no Bus controller. Data transfer is Simplex, that is it only travels in one direction. This illustration is drawn with outputs represented exiting the side of the boxes, and inputs entering the top or bottom. This wiring network looks more complex than the 1553 data bus because data transfer is only simplex or one way. But the ARINC 429 data bus system does not require a Bus Controller. Because all data outputs are transmitted over two output wires, wiring is still kept to a minimum, eg 2 wires carry Indicated Airspeed, True Airspeed, Mach Number, Altitude, AOA, OAT, and Built In Test Data. If two components need to exchange information, eg the Flight Controls Computer (FCC) sends flight control surface position information to the Flight Management Computer (FMC), and the FMC transmits autopilot commands to the FCC. In just this case, the FCC output is sent to the FMC, and the FMC output is sent to the FCC. Following is a list of the basic signal transfer considered in developing this diagram for instructing purposes: Flight Control Computer (FCC) Output Surface Positions and maintenance data sent to FMC for eventual display on an MFD Failure monitoring output sent to EICAS, eg Aileron or Flap FCC Channel Fail Air Data Computer (ADC) Output Altitude, Airspeed, AOA, Total Temp, etc sent to: •

FCC for gain scheduling

•

TMC for thrust management calculations

•

FMC for eventual display

Failure monitoring output sent to EICAS, eg AOA sensor fail





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Internal Reference System (IRS) Output Accelerometer outputs detecting yaw, or sideslip sent to TMC to maintain symmetrical flight Roll, Pitch, Yaw to FCC for autopilot, and to FMC for display on an MFD Failure monitoring output sent to EICAS, eg sensor overheat Thrust Management Computer (TMC) Output To FMC for display on an MFD; EPR, EGT, etc Failure monitoring output sent to EICAS eg oil pressure low, N2 RPM high Flight Management Computer (FMC) Output Sent to all avionics components for overall system control utilising FMC display/select panel Engine Indicating and Crew Alerting System (EICAS) In this diagram EICAS has no outputs to other avionic systems, it purely monitors inputs. Because it has no outputs it has no need of an ARINC 429 output data bus.

ARINC 429 Specifications Each aircraft may be equipped with different electronic equipment and systems needing interconnection. A large amount of equipment may be involved depending on the aircraft. These are identified in the specification and are assigned digital identification numbers called Equipment ID. A partial list of equipment identified in ARINC Specification 429 is illustrated below along with their digital addresses.

ARINC 429 details a significant amount of specifications, all of which are intended to set a standard for avionics data bus systems installed in aviation industry equipment.





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ARINC 429 DATA TRANSFER The Manchester II Bi-Phase coding of the 1553 data bus transferred data (1’s & 0’s) by the shifting polarity of a signal (+ve to –ve = 1and –ve to +ve = 0) , and hence was not reliant upon voltage levels. The ARINC 429 data bus uses RTZ or return to zero where no signal is relayed by a signal voltage of 0

ARINC 429 DATA TRANSFER The intricacies of the method of data transfer doesn’t really matter to us, as long as you understand the concept, and realise that both data bus systems (1553 & ARINC 429) have different signalling methods. The fact that signals are transferred by different methods provides evidence of the flexible nature of digital comms, & as long as all components are speaking the same language and are synchronised communication will result.

ARINC 429 is a very simple, point-to-point protocol. There can be only one transmitter on a wire pair. The transmitter is always transmitting either 32-bit data words or the NULL state. There may be up to 20 receivers on a wire pair. In most cases, an ARINC message consists of a single data word. The label field of the word defines the type of data that is contained in the rest of the word. Typically, messages are sent repetitively. For example, measured airspeed is transmitted from the sensor to the instrument at intervals not less than 100 milliseconds or greater than 200 milliseconds. Messages may also be sent in repetitive word sequences or frames. Messages from each fuel tank level sensor are sent in sequence, and then the sequence is repeated after a specified time. Once the 63-word sequence to relate all the fuel tank levels is completed, it repeats, starting over with word 1. Most of the data is in BNR format, but some words are in BCD. Communications on 429 buses use 32-bit words with odd parity. A low speed-bus (12 to 14.5 kilobits/sec) is used for general-purpose, low-criticality application, and a high-speed bus (100 kilobits/sec) is used for transmitting large quantities of data or flight critical information.





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ARINC 429 Words ARINC Specification 429 specifies amongst other things the codes used as identifying labels for instructions and the standard types of data used in an aircraft multiplexing system. It also specifies that the information from the output port of an avionics system element (eg Navigation Computer) be “communicated” over a single twisted and shielded pair of wires to all other systems elements requiring the information. This means the information protocol is specified in ARINC 429, detailing standard data codes (labels) and formats which are to be incorporated into the data transfer network. LABEL – The label is the first 8 bits of a word and identifies the data type and the parameters associated with it. The label is an important part of the message, it is used to determine the data type of the remainder of the word and, therefore, the method of data translation to use. Labels are typically represented as octal numbers. SDI – Bits 10 and 9 provide a Source/Destination Identifier or SDI. This is used for multiple receivers to identify the receiver for which the data is destined. It can also be used in the case of multiple systems to identify the source of the transmission. In some cases, these bits are used for data. ARINC 429 can have only one transmitter on a pair of wires, but up to 20 receivers.

Data – Bits 29 through 11 contain the data, which may be in a number of different formats. Some examples are provided later in the tutorial. There are also many non-standard formats that have been implemented by various manufacturers. In some cases, the data field overlaps down into the SDI bits. In this case, the SDI field is not used. SSM – Bits 31 and 30 contain the Sign/Status Matrix or SSM. This field contains hardware equipment condition, operational mode, or validity of data content. This refers to Plus, Minus, North, South, Left, Right, etc of the binary coded decimal (BCD) numeric data. It can also refer to the validity of the data and a failure warning. Parity – The parity bit functions as explained earlier, for odd parity the parity bit will be a 1 if there are an even number of 1’s in the preceding part of the data word. The parity bit will be a zero if there are already an odd number of 1’s in the word. This means there will always be an odd number of ones in each data word, when the parity bit is included





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ARINC 429 Data Types Although the Label, SDI, SSM, and Parity bits remain somewhat fixed in each word, bits 11 – 29 which contain the data may be laid out in several formats. Some data is sent in BCD format, where each four binary bits represent a decimal number, this could be a word transmitted to an MFD for display of Altitude, eg 25 786 feet. Remember it is the label identifying the type of data that advises the receiving unit of which format the data is sent in, hence the importance of the label. The “label” and the data encoding method is laid down in ARINC 429.

In the second example a purely binary value is transmitted 0 100 001 100 000 000 000 which equals 13721610 (base 10). This could represent total fuel remaining in lbs, again the label identifies the data contained & what it is encoded in. Data label & encoding is laid down in ARINC 429. All ARINC data is transmitted in 32 bit words. The data type may be Binary Coded Decimal (BCD), two’s complement, binary notation (BNR), Discrete Data, Maintenance Data and Acknowledgment, & ASCII. ARINC incorporated ASCII (American Standard Code for Information Interchange) code into ARINC Specification 429. ASCII code is accepted world wide as the “International Standard Organisation Code No. 5 (ISO No. 5). It translates all alphanumeric inputs from the keyboard into the representing binary numbers required by the system.





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ARINC 429 Data Types There are several basic word formats in 429, for numerical data, discretes, and alphanumeric data, which are encoded using International Standards Organization (ISO) Alphabet No.5 (ASCII). All are based on the standard arrangement with a label, SSM and parity, but there are minor variations between them depending on the data being transmitted. The label indicates the data type so the RT can interpret what is sent.





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ARINC 429 Labels ARINC Specification 429 specifies amongst other things the codes used as identifying labels for instructions and the standard types of data used in an aircraft multiplexing system. It also specifies that the information from the output port of an avionics system element (eg Navigation Computer) be “communicated” over a single twisted and shielded pair of wires to all other systems elements requiring the information. This means the information protocol is specified in ARINC 429, detailing standard data codes (labels) and formats which are to be incorporated into the data transfer network. The label is the first 8 bits of a word and identifies the data type and the parameters associated with it. The label is an important part of the message, it is used to determine the data type of the remainder of the word and, therefore, the method of data translation to use. Labels are typically represented as octal numbers.

Labels may be associated with more than one equipment type, and the equipment IDs associated with the examples are illustrated on the slide. Thus BCD label 010 is always present latitude, but it can pertain to three different sources, the Flight Management Computer (002), the Inertial Reference System (004), or ADIRS (038). BCD label 014 is either Magnetic Heading from the Inertial Reference System (004), Attitude and Heading Reference System (005), or ADIRS (038). This examples also provides additional specifications for the data transmissions, number of digits used to transfer the data, for present position a +ve data signal (SSM or just a positive digital number) indicates latitude North, whereas a negative number indicates latitude South. Data transmission rate means the parameter is transmitted from the source at a minimum of once every 250 ms (4 / sec) up to a max of once every 500 ms (twice per second).





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ARINC 629 History The new technology, called Digital Autonomous Terminal Access Communication (DATAC), was originally being developed at Boeing. It was a design for a single, global data bus that would carry all the information between the different components of the airplane systems. The data bus consisted of a single twisted pair of wires, to which all the components that needed to exchange information were connected. To keep the data from each component from getting jumbled with the other information being exchanged, each component's data was coded and "broadcast" in a synchronised order. All the information was transmitted on the data bus, and each computer or component could be programmed to pull off whatever information it needed. The DATAC system was a perfect design for the NASA 737. It would allow a far greater number of components to be integrated into the aircraft systems, and it would greatly reduce the amount of time required to add or exchange experimental equipment. Since the data bus had fewer wires and components, it would also be lighter and would require less maintenance than a conventional system. By the beginning of August 1984, DATAC was installed and working in the airplane. The 737 made an excellent test bed for a new data bus, because the equipment in the front cockpit remained conventional. Later on, the DATAC technology was incorporated into Boeing's next jet transport design, the B777. DATAC worked so well, in fact, that Aeronautical Radio, Inc. (ARINC) used it as the basis of a new industry data bus standard. ARINC is a notforprofit organization owned and supported by the airlines that sets standard specifications for technology, so the products developed by different manufacturers will be compatible with all commercial transport aircraft. The specification for the new data bus, called ARINC 629, was adopted in September1989..

ARINC 629 Interconnection The ARINC 629 data bus is a time division multiplex system. It is a bidirectional, distributed control bus capable of supporting up to 120 users at a transmission rate of 2 Mbps. It includes multiple transmitters with broadcast—type, autonomous terminal access. Terminal access is autonomous. Terminals listen to the bus and wait for a quiet period before transmitting. The users communicate with the bus using a coupler and terminal. Only one terminal is allowed to transmit at a time. After a terminal has transmitted, three different protocol timers are used to ensure that it does not transmit again until all of the other terminals have had a chance to transmit. The figure shows the interconnection of two systems using ARINC 629. In this example, the ARINC 629 terminal controller and Serial Interface Module (SIM) are installed on a circuit board





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within each LRU. The SIM interfaces with the stub cable via a connector on the LRU. The stub cable is then coupled to the global data bus via a current mode coupler.

ARINC 629 Interfaces The ARINC 629 data bus system has these parts: •

Data bus cable

•

Current-mode couplers

•

Stub cables

The ARINC 629 system also includes these components in the LRUs: •

Serial interface modules

•

Terminal controllers

The current-mode coupler connects the bus cable to the stub cable. The stub cables are for bidirectional data movement between the LRU and the current-mode coupler. The stub cables





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also supply power from the LRUs to the current-mode couplers. A stub cable has four wires; two to transmit and the other two to receive. An ARINC 629 LRU contains a serial interface module (SIM) and a terminal controller. These move data between the LRU and the current-mode coupler. Each LRU has a personality that identifies its purpose and operation. The personality data is in two parts: •

Transmit personality PROM (XPP)

•

Receive personality PROM (RPP)

The terminal controller uses the personality proms to control the flow of data between the LRU and the data bus. ARINC 629 Data bus cable The bus cable moves data between LRUs. A current-mode coupler and a stub cable attach each LRU terminal to the data bus cable. A bus cable is a pair of twisted wires with a termination resistor at each end. Each resistor has a value of about 130 ohms. The left and right systems bus cables have production break connectors in the middle for easy replacement. The parts of the system bus cable that are external to the coupler panels have shielding outside. A bus cable in the 777 may be as long as 180 feet. It connects as many as 46 current-mode couplers. The cable has a center conductor covered by a layer of foam. A Teflon skin covers the foam.

ARINC 629 Message Structure Data in the ARINC 629 system moves through the data bus cable and other components as messages. Between each message is a terminal gap (TG). A message is a group of word





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strings. Each word string has a label word followed by data words. Each message has a special structure. The structure allows the LRUs to select and read the message. Message Structure A message has up to 31 word strings. There is a 4-bit time gap between each word string. A word string begins with a label word. A word string has up to 256 data words. There is no gap between words in a word string. The minimum length message has 1 label and no data words. The maximum length message has 31 labels with 256 data words following each label, and 30 time gaps of 4 bits each. Label Word Structure A label word is a 20-bit word. It has: •

A 12-bit label field

•

A 4-bit label extension field

•

A single parity bit

•

A 3-bit time hi lo sync pulse

A pulse of one half-bit time, called the pre-sync sync pulse (PSSP), comes before the first label word of a message. An approximately one half-bit time pre-pre-sync sync pulse (PPSSP) comes before the PSSP. The PPSSP and the PSSP occur prior to the 3-bit time hi lo sync pulse. Data Word Structure A data word is also a 20-bit word. It has: •

A 16 bit data field

•

A single parity bit

•

A 3-bit time lo hi sync pulse





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ARINC 629 Timing Because of the quantity of data that may be on the bus, ARINC 629 uses a time procedure to prevent accidental signal mixture. ARINC 629 uses three timers: •

Transmit interval (TI) timer

•

Synchronization gap (SG) timer

•

Terminal gap (TG) timer

The timers are part of the LRU personality. Each LRU uses all three timers to isolate data messages. Transmit Interval (TI) The TI for any LRU begins the moment the terminal starts to transmit. After the terminal transmits a message, it must wait the length of time equal to TI before it transmits again. All LRUs on a bus have the same TI. Synchronization Gap (SG) After the TI, the SG is the longest timer. The SG begins when there is no signal on the bus. The SG is the same for all LRUs. It has a value larger than the value of the longest terminal gap used on a given bus. If a signal comes on the bus before the SG completes, the SG stops. When the SG completes, it stays reset until the LRU transmits again. Terminal Gap (TG) Each LRU on the bus has a special TG. The TG begins after the SG is complete and no signal is on the bus. If there is a signal on the bus before the TG completes, the TG stops. It starts again when there is no signal on the bus. The TG and SG cannot overlap in time. They must occur in sequence.





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ARINC 629 Timing Mode The ARINC 629 three timers operate in these two ways: •

Periodic mode

•

Aperiodic mode

The periodic mode makes sure that an LRU transmits at a regular time sequence, in their powerup order. If an LRU message length increases because of a non-normal condition, the system changes to aperiodic operation. In aperiodic operation, the LRU transmits in a different time sequence. In the aperiodic mode, the LRUs transmit in order of shortest TG to longest TG. Periodic Mode The periodic mode is the normal mode of operation. In the periodic mode, an LRU transmits one time every TI. The examples show the timing diagram for three LRUs in the periodic mode.

At event 1, all three timers (TI, SG & TG) for LRU 1 are complete and LRU 1 starts to transmit a message (M). LRUs 2 and 3 stop their TGs when LRU 1 starts to transmit. At event 2, LRU 1 no longer transmits, and LRU 2 and 3 start their TG timers. At event 3, the TG timer for LRU 3 is complete, however TI still continues. LRU 3 does not transmit. The TG timer for LRU 2 continues. At event 4, the TG timer for LRU 2 is complete. All three timers for LRU 2 (TI, SG and TG) are complete and the LRU 2 starts to transmit a message, while LRU 3 waits for its TI to complete. LRU 3 stops its TG when LRU 2 starts to transmit. At event 5, LRU 2 stops transmission and LRU 2 starts its TG timer. At event 6, the TG timer for LRU 3 completes. For LRU 3, all three timers (TI, SG and TG) are complete and it starts to transmit a message. At event 7, LRU 3 no longer transmits, and all three LRUs start their SG timers. At event 8, all three SG timers are complete and the TG timers start. At event 9, the TG for LRU 1 completes, TI continues, so it does not transmit. At event 10, TG for LRU 3 is complete, the TI continues, so the LRU does not transmit. At event 11, the TG for LRU 2 completes, the TI continues, so the LRU does not transmit. Back at event 1 when all three timers (TI, SG and TG) for LRU 1 are complete and it starts to transmit a message. LRU 2 and 3 stop their TGs when LRU 1 starts to transmit.





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Aperiodic Mode If the sum of all the TGs, transmission times and SG is greater than TI then the system operates in aperiodic mode. Aperiodic data is a direct result of a discrete event. It is data that is asynchronous and updated at a non-uniform rate. For example, aperiodic data can be a position report in landing gear systems. Aperiodic data transfers data on events important to airplane operation (In this example, TG1
Data to control tasks such as landing gear sensors and flight deck switches

•

Data for status information

Aperiodic data also transmits large blocks of data for these functions: •

Data base loads

•

Operational software

•

BITE information





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Data Bus Summary





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TOPIC 5.5.1 – LOGIC CIRCUITS Basic Gates These are the basic gates. The NAND and NOR gates are known as universal gates. That is ALL gates can be constructed from them.

AND Gate The output is high only when both inputs A and B are high. Some logic gates can be produced with just diodes and resistors (called diode resistor logic or DRL).

Consider that if the input to an input diode is NOT a HI, then it is a LO. If one of the inputs A or B is grounded, current flows through the diode and the output is at a low voltage. The only way to get a high output is by having both inputs high. This is clearly a logical AND function.


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Topic 5.5.1: Logic Circuits Page 1 of 16


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The illustrated equivalent circuit uses 2 NPN transistors connected in series. These are designed to operate as a switch (ie. In saturation). The 10 K resistors are for input impedance and the 4.7 K resistor is to drop output signal across. Note that both transistors must be ON to get a HI output. Above left illustration is an example of convenient packaging of AND gates in integrated circuit form (IC7408 AND Gates).


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OR GATE The output is high when either or both of inputs A or B is high. Some logic gates can be produced with just diodes and resistors (called diode resistor logic or DRL). Whenever one or the other of the inputs A and B are high, current flows through the associated diode. This brings the output to a high voltage. This circuit clearly implements a logical OR.

Also note that it is not possible to construct an inverter with only diodes and resistors. AND and OR functions by themselves are not a complete logic without NOT. Thus, there are some logic functions that cannot be implemented in diode-resistor logic. Fortunately, transistors solve all of these problems.

The illustrated equivalent circuit uses 2 NPN transistors connected in parallel with Vcc. These are designed to operate as a switch (ie. In saturation). Note that with either transistor ON, the output will be HI.


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NAND Gate

The output is high when either of inputs A or B is high, or if neither is high. In other words, it is normally high, going low only if both A and B are high. The NOR gate and the NAND gate can be said to be universal gates since combinations of them can be used to accomplish any of the basic operations and can thus produce an inverter, an OR gate or an AND gate. The non-inverting gates do not have this versatility since they can't produce an invert.


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NOR Gate

The output is high only when neither A nor B is high. That is, it is normally high but any kind of non-zero input will take it low. The use of transistors for the construction of logic gates depends upon their utility as fast switches. When the base-emitter diode is turned on enough to be driven into saturation, the collector voltage with respect to ground may be less than a volt and can be used as a logic 0 in the TTL logic family.


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XOR Gate – Exclusive OR Gate

The output is high when either of inputs A or B is high, but not if both A and B are high. Logically, the exclusive OR (XOR) operation can be seen as either of the following operations: 

A AND NOT B OR B AND NOT A



A OR B AND NOT A AND B

which can be implemented by the gate arrangements shown. They can also be implemented using NAND gates only.

The implementation of the exclusive OR (XOR) operation with just NAND gates illustrates the function of NANDs as universal gates.

XNOR Gate – Exclusive NOR Gate

The output is high when both inputs A and B are high and when neither A nor B is high. The exclusive NOR is an exclusive OR followed by an inverter. The illustration is one way to make it using basic gates.


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Buffer

The buffer is a single-input device which has a gain of 1, mirroring the input at the output. It has value for impedance matching and for isolation of the input and output. The basic emitter follower can be used as a buffer. The common collector amplifier is often called an ‘emitter follower’ since its output is taken from emitter resistor. It is useful as an impedance matching device since its input impedance is much higher than its output impedance.

An op-amp voltage follower can serve as a buffer. The voltage follower with an ideal op amp gives simply Vout = Vin, but this turns out to be a very useful service, because the input impedance of the op amp is very high, giving effective isolation of the output from the signal source. You draw very little power from the signal source, avoiding "loading" effects. This circuit is a useful first stage. The voltage follower is often used for the construction of buffers for logic circuits.


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Inverting Buffer (Inverter)

The inverting buffer is a single-input device which produces the state opposite the input. If the input is high, the output is low and vice versa. This device is commonly referred to as just an inverter. A transistor switch with collector resistor can serve as an inverting buffer. When the switch is open, no current flows in the base so the collector current is cut off. The resistor Rc must be small enough to drive the transistor to saturation so that most of the voltage Vcc appears across the load. The output is taken below the load resistor and can function as an inverting buffer in digital circuits.

An op-amp inverting amplifier with a gain of one serves as an inverting buffer. For an ideal op-amp, the inverting amplifier gain is given simply by:

For equal resistors, it has a gain of -1, and is used in digital circuits as an inverting buffer.


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Inverter Symbol The inverter symbol is utilised as required, but a more common method of indicating a signal is inverted is simply by the use of an inversion symbol an ‘O’ on the leg of the device.

IEEE Gate Symbols

Together with the American National Standards Institute (ANSI), the Institute of ANSI Electrical and Electronic Engineers (IEEE) has developed a standard set of logic IEEE symbols. The most recent revision of the standard is ANSI/IEEE Std 91-1984, IEEE Standard Graphic Symbols for Logic Functions. It is compatible with standard 617 of the International Electrotechnical Commission (IEC), and must be used in all logic diagrams drawn for the U.S. Department of Defense. These symbols are being used more and more as time progresses.


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Universal Gates

The NOR gate and the NAND gate can be said to be universal gates since combinations of them can be used to accomplish any of the basic operations and can thus produce an inverter, an OR gate or an AND gate. The non-inverting gates do not have this versatility since they cannot produce an invert.

Logic Circuits Logic circuits are used in electronic devices. Logic circuits are formed by combining many logic gates. More complex logic circuits are assembled from simpler ones which in turn are assembled from gates. The building block of ALL logic circuits is the logic gate.

Combining Gates Together Multiple input gates can be constructed by placing gates in special configurations.

A 3 input AND gate may be constructed using 2 AND gates connected as shown. A 3 input OR gate may be constructed using 2 OR gates connected as shown.


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Logic with Waveforms The logic waveforms illustrated are applied to the inputs of both an OR and a XOR gate.

Note that logic level 1 (HIGH) is represented by +5V and similarly, logic level 0 (LOW) is represented by 0V.


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Logic Circuit Representations

Many circuits in aircraft may be schematically represented by logic circuits for ease.


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Applications of Logic Circuits in Aircraft Systems This is a typical applications of a logic gates/circuit used in aircraft systems. This is from the A320 Airbus Hydraulic Power system. This application is for the automatic operation of the BLUE Hydraulic system Electrical Pump. An OR gate is used in this instance. The BLUE electric pump will automatically start with any of the following conditions: 

Engine 1 running (>50%)



Engine 2 running (>50%)



Nose gear shock absorber extended

The operation of the system is fully automatic. If necessary (because of fault or maintenance), it is possible to stop the automatic operation of the system. The BLUE electric pump automatically starts and supplies the system when any of the engines starts. If the 2 engines stop in flight, the BLUE system electric pump will continue to operate if the AC power supply comes from the APU. In this case, compression of the nose landing gear stops the electric pump. A time delay circuit keeps the electric pump in operation for 2 minutes after compression of the nose landing gear. This ensures that the electric pump does not stop immediately when the aircraft lands.


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Applications of Logic Circuits in Aircraft Systems This is a typical application of a logic circuit shown as a schematic. It may not be like this in reality, but will operate like this. It may be drawn in this manner to assist engineers. The system itself may be also be software controlled although it may be represented like this for ease.


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TOPIC 5.6.1 – BASIC COMPUTER STRUCTURE Computer Terminology Transistors are turned ON & OFF by applying a voltage to the base, computers function utilising only two logic levels, “ON” and “OFF”. This then is why we must understand the binary numbering system, which only uses two logic levels, 0 and 1, which represent the transistors being “ON” or “OFF”. A 1, or ON or a HIGH is typically about 5VDC, and a 0, or OFF or a LOW is typically 0VDC.


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A bit of information is the 1 or 0 applied to the transistors, gates, or integrated circuits of the computer system.

The transistor circuit on the right is a simple AND gate. To get the light ON you must apply a high or 5VDC to both the transistors. In this way logic can be used to perform function, the light will only swith ON if both inputs are high. This is a foundation to all computer processing. Circuit on the left is also an AND gate. If SW 2 is turned ON, upper transistor forward biases, but current flows through the resistor in parallel with the bottom transistor, so there is still no change in state measured at the output by the voltmeter. Both SW 1 & SW 2 must be turned on too forward bias both transistors to produce a change in state at the output.


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Bit’s, Nibbles and Bytes A bit is not a large enough vehicle to carry much information. With one bit, there are only two possible values, 0 and 1. With two bits, there are four possible values: 00, 01, 10, 11. With 3 bits, there are 8 possible values: 000, 001, 010, 011, 100, 101, 110, 111. With 4 bits (a nibble) there are 16 possible values, With 8 bits there are 256 possible values, and so on... 8 bits of information are called a BYTE, (4 bits are called a nibble). Bit

1

Nibble 0101 Byte

0000 0101

A Byte of information can output 256 possible combinations of bits, and is of far more use in designing computer components and modules than a bit. A single character on a keyboard is represented internally as a series of 8 bits, or a byte.

Computers typically store and manipulate bunches of eight bits of data. These eight bits make up a byte. Both EBCDIC and ASCII codes use eight bits to represent a single character, such as the letter A or the number 7. Thus, the computer can store and manipulate an individual byte (a single character) or a group of bytes (several characters, a word) at a time. These individual bytes, or groups of bytes, form the basic unit of memory. Primary storage capacities are usually specified in number of bytes. The symbol "K" is used whenever we refer to the size of memory, especially when the memory is quite large. The symbol K is equal to 1,024 units or positions of storage. Therefore, if a computer has 512K bytes (not bits) of primary storage, then it can hold 512 × 1,024 or 524,288 characters (bytes) of data in its memory.


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Integrated Circuits Gates are constructed utilising transistors as the switching elements. Gates can then be arranged to perform calculations and processes when configured for the task.

In this example a range of gates are configured to produce an adder. The adder which is a series of gates is then represented by a single symbol, and can be configured into a series of adders. As components and integrated assemblies are further combined and arranged, eventually a computer is constructed. The illustration on the slide is not a representation of the gate circuit – it is provided to illustrate the concept only. In a computer, single bit’s of information are rarely referred to, but every function performed by a computer is done at the most basic level by applying a bit of information to a transistor.


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Data Processing Circuits Transistorised Gates can be linked together to produce circuits which are capable of performing calculations. When you type numbers into a computer to perform calculations they are converted to binary, calculated in the processing circuitry, and the output is converted back into decimal for you to read. The circuits which perform the calculations are simply logic gates connected so as to produce the desired outcome. A half adder is used to add two binary digits together. Although by itself, a half adder is not very useful, it can be used as a building block for larger adding circuits. The truth table reflects the sum of the two inputs. When adding binary digits, it is always possible to have a carry. Therefore, any circuit which is going to add must have a line for carrying on any overflow. A full adder is used to add three binary digits together. The easiest way to build a full adder is to use two half adders. This construction is the reason for the names of the two types of adders: a full adder is called "full" because all possible values of output can be achieved. While a half adder can only output 00, 01, & 10, a full adder can also output 11. A half adder is half of a full adder, since it takes two half adders (and an OR Gate) to make a full adder. A half adder can be simply represented as a box with "HA" written in it, just as integrated circuits are constructed, in the same way a full adder can be represented as a box with "FA" written in it. This is typical of almost all electronic applications, rarely are circuits drawn showing electron flow paths and discrete components (although that is what they contain) they are simply represented by a bow with inputs and outputs labelled.

A simple system of four microchips that contain sufficient AND, OR, and NOT gates to add two two-place binary numbers. As shown, an input of binary "10" + binary "11" produces a binary output of "101" (or as expressed in decimal terms: 2 + 3 = 5). These simple logic gates multiplied many-fold are the heart and soul of all calculators and computers, whether they are processing numbers or alphabetical letters.


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Integrated Circuits – Construction Up to now the various semiconductors, resistors, capacitors, etc., in our discussions have been considered as separately packaged components, called DISCRETE COMPONENTS. Some of the more complex devices contain complete circuits packaged as a single component. These devices are referred to as INTEGRATED CIRCUITS and the broad term used to describe the use of these devices to miniaturise electronic equipment is called MICROELECTRONICS active components (transistors, diodes, etc.) and passive components (resistors, capacitors, inductors, etc.)

With the advent of the transistor and the demand by the military for smaller equipment, design engineers set out to miniaturise electronic equipment. In the beginning, their efforts were frustrated because most of the other components in a circuit such as resistors, capacitors, and coils were larger than the transistor. Soon these other circuit components were miniaturised, thereby pushing ahead the development of smaller electronic equipment. Along with miniature resistors, capacitors, and other circuit elements, the production of components that were actually smaller than the space required for the interconnecting wiring and cabling became possible. The next step in the research process was to eliminate these bulky wiring components. This was accomplished with the Printed Circuit Board (PCB). An integrated circuit is a device that integrates (combines) both active components and passive components of a complete electronic circuit in a single chip (a tiny slice or wafer of semiconductor crystal or insulator). Integrated circuits (ICs) have almost eliminated the use of individual electronic components (resistors, capacitors, transistors, etc.) as the building blocks of electronic circuits. Instead, tiny chips have been developed whose functions are not that of a single part, but of dozens of transistors, resistors, capacitors, and other electronic elements, all interconnected to perform the task of a complex circuit. Often these comprise a number of complete conventional circuit stages, such as a multistage amplifier, logic circuits linear circuits and operational amplifiers (in one extremely small component). These chips are frequently mounted on a printed circuit board which plugs into an electronic unit.


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This integrated circuit was produced in about 1960 by Fairchild Semiconductor. It is a bistable RS (Reset/Set) Flip-Flop constructed using four NPN bipolar transistors and two resistors diffused into a single monolithic chip of silicon. The maximum operating clock speed is 1 megahertz and the delay is 50 nanoseconds Integrated circuits have several advantages over conventional wired circuits of discrete components. These advantages include (1) a drastic reduction in size and weight, (2) a large increase in reliability, (3) lower cost, and (4) possible improvement in circuit performance. However, integrated circuits are composed of parts so closely associated with one another that repair becomes almost impossible. In case of trouble, the entire circuit is replaced as a single component.

Integrated circuits are being used in an ever increasing variety of applications. Small size and weight and high reliability make them ideally suited for use in airborne equipment, computers, spacecraft, and portable equipment. They are often easily recognized because of the unusual packages that contain the integrated circuit. These tiny packages protect and help dissipate heat generated in the device. One of these packages may contain one or several stages, often having millions of components.


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Digital Computers Personal computers will be more familiar to you, and the operating principles of personal computers relate directly to the operating principles of avionics computers. If we were to define the word computer, we would say a computer is an instrument for performing mathematical operations, such as addition, multiplication, division, subtraction, integration, vector resolution, coordinate conversion, and special function generation at very high speeds. But the usage of computers goes well beyond the mathematical-operations level. Computers have made possible aviation, scientific, and commercial advances that before were considered impossible. The mathematics involved in orbiting a satellite around the earth, for example, would require several teams of mathematicians for a lifetime. Now, with the aid of electronic digital computers, the conquest of space has become reality. Computers are employed when repetitious calculations or the processing of large amounts of data are necessary. The most frequent applications are found in the aviation, scientific, and commercial fields. They are used in many varied projects, ranging from mail sorting, through engineering design, to the navigation of an aircraft around the world. The advantages of digital computers include speed, accuracy, reliability, and man-power savings. The First useful digital computer was the ENIAC, developed in 1945. It weighed 30 tons, took up 1,800 square feet, and used vacuum tubes. It required a staff of 6 just to keep it going, and it cost over $500,000 to build.

The first personal computer of note was the Altair 8800 (1975), which was actually a kit you had to build yourself. You also had to but "peripheral" items like a keyboard from separate companies. The cost was $395, and 2000 were sold, more than any computer before it. Instructions can be entered with switches at the front and then run.

The Apple II was introduced in 1977, it came built and self-contained, and cost $1298. It introduced some radical features, such as colour graphics. Like many of the early PCs, the Apple II used a cassette tape interface for storing data/programs. Commodore PET microcomputers were also of this era The IBM PC was introduced in 1981, and it soon led to the microcomputer industry boom. The first PC was based on the 8088 processor, & had 64 KB of RAM, and a 5.25" floppy drive. It cost $3000. The monitor was extra.


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Central Processing Units A computer processor is a huge collections of circuits similar to the adders described, and designed to produce a particular output for certain inputs. A calculator is one of the simplest processors, it’s equivalent of a Central Processing Unit (CPU) is designed to only deal with numbers, and to just perform algebraic calculations.

A modern computer CPU performs much more complex calculations, but is still nothing more than a collection of logic circuits configured to produce a desired outcome. The brain of a computer system is the central processing unit, in fact THE CPU IS THE COMPUTER which we generally refer to as the CPU or mainframe. It is the CPU that processes the data transferred to it from one of the various input devices, and then transfers results of the processing to one of many output devices. The inputs can be on any storage medium from punched cards, paper tape, or magnetic ink to magnetic tape, disk, or drum; or they can be entries from a console keyboard or a visual display. The output may be in punched cards or paper tape, on magnetic tape, disk, or drum, or it may be printed reports or information displayed on a console typewriter or visual display. A central control section and work areas are required to perform calculations or manipulate data. Inputs are provided to the CPU from and number of sources, The CPU performs calculations as directed by the software program resident in its memory, and produces an output which is then transmitted to whatever medium is required. The main point is that it is the CPU which performs the magic, all other electronic components interfaced to the CPU perform functions as designed (video cards, sound cards, memory, etc) but it is the CPU which pulls the whole thing together.


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Control Section The control section may be compared to a telephone exchange because it uses the instructions contained in the program in much the same manner as the telephone exchange uses telephone numbers. When a telephone number is dialled, it causes the telephone exchange to energise certain switches and control lines to connect the dialling phone with the phone having the number dialled. In a similar manner, each programmed instruction, when executed, causes the control section to energise certain control lines, enabling the computer to perform the function or operation indicated by the instruction. The program is usually stored in the internal circuits of the computer (computer memory) as the BIOS or operating system. In addition to the commands that tell the computer what to do, the control unit also dictates how and when each specific operation is to be performed. It is also recalls relevant data from memory and moves it to the appropriate point where the processing can be accomplished. The four major types of instructions are (1) transfer, (2) arithmetic, (3) logic, and (4) control. Transfer instructions are those whose basic function is to transfer (move) data from one location to another. Arithmetic instructions are those that combine two pieces of data to form a single piece of data using one of the arithmetic operations. Logic instructions transform the digital computer into a system that is more than a high-speed adding machine. Using logic instructions, the programmer may construct a program with any number of alternate sequences. For example, through the use of logic instructions, a computer being used for maintenance inventory will have one sequence to follow if the number of a given item on hand is greater than the order amount and another sequence if it is smaller. The choice of which sequence to use will be made by the control section under the influence of the logic instruction. Logic instructions, thereby, provide the computer with the ability to make decisions based on the results of previously generated data. That is, the logic instructions permit the computer to select the proper program sequence to be executed from among the alternatives provided by the programmer. Control instructions are used to send commands to devices not under direct command of the control section, such as input/output units or devices.

Arithmetic-Logic Section The arithmetic-logic section performs all arithmetic operations-adding, subtracting, multiplying, and dividing. Through its logic capability, it tests various conditions encountered during processing and takes action based on the result. As indicated by the solid arrows on the illustration on the slide, data flows between the arithmetic-logic section and the internal storage section during processing. Specifically, data is transferred as needed from the internal storage section to the arithmetic-logic section, processed, and returned to the internal storage section. At no time does processing take place in the storage section. Data may be transferred back and forth between these two sections several times before processing is completed. The results are then transferred from internal storage to an output unit, as indicated by the solid arrow.


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Memory (internal storage) Section The memory (internal storage) section of a computer is essentially an electronically operated file cabinet. It has a large number (millions, eg gigabytes) of storage locations; each referred to as a storage address or register. Every item of data and program instruction read into the computer during the loading process is stored or filed in a specific storage address and is almost instantly accessible. All memory sections must contain facilities to store computer data or instructions until these instructions or data are needed in the performance of the computer calculations. Before the stored-program computer can begin to process input data, it is first necessary to store in its memory a sequence of instructions, and tables of constants and other data it will use in its computations. The process by which these instructions and data are read into the computer is called loading. Actually, the first step in loading instructions and data into a computer is to manually place enough instructions into memory using the keyboard or electronically using an operating system, so that these instructions can be used to bring in more instructions as desired. In this manner a few instructions are used to bootstrap more instructions.

Operating System The operating system defines our computing experience. It's the first software we see when we turn on the computer, and the last software we see when the computer is turned off. It's the software that enables all the programs we use. The operating system organizes and controls the hardware. An operating system is a collection of many programs used by the computer to manage its own resources and operations. These programs control the execution of other programs. They schedule, assign resources, monitor, and control the work of the computer. Not all computers have operating systems. The computer that controls a microwave oven, doesn't need an operating system. It has one set of relatively simple tasks to perform, very simple input and output methods (a keypad and an LCD screen), and simple, never-changing hardware to control. For a computer like this, an operating system would be unnecessary baggage, adding complexity where none is required. Instead, the computer in a microwave oven simply runs a single program all the time. For computer systems that go beyond the complexity of the microwave, however, an operating system can be the key to greater operating efficiency and easier application development. All desktop computers have operating systems. The most common are Windows, Unix & Macintosh, but hundreds are available depending on the application utilising the operating system, eg mainframes, robotics, manufacturing, real-time control systems and so on. At the simplest level, an operating system does two things: It manages the hardware and software resources of the computer system. These resources include such things as the processor, memory, disk space, etc. It provides a stable, consistent way for applications to deal with the hardware without having to know all the details of the hardware. An operating system is simply a set of programs and routines that lets you and other programs use the computer. A digital computer uses one central set of programs called the operating system to manage execution of other programs and to perform common functions like read, write, or print. Other programs, or you the user, can order the operating system to perform these common functions. These orders are called system calls when other programs use them, or simply commands when you put them through the keyboard. Each desktop computer has a built-in program called "bootstrap loader." When you turn the computer on, this program tries to load, or "boot," an external operating system. The term boot comes from the idea of pulling yourself up by your bootstraps. The computer loads a little program from the disk that tells it how to load a second, bigger program (the operating


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system). The operating system then tells it how to load another program (an applications program or utility program) to perform a specific job or function. In physical terms the CPU is the turnstile through which everything must pass. Regardless of how much information you have to process, it is the turnstile which is the overriding factor in how quickly the data is processed.

Top Illustrations are of an 8088 processor, lower ones are the 80286 processor. When a computer is referred to it is typically with reference to the CPU. The IBM PC was introduced in 1981, its CPU was based on the 8088 processor, and had 64 KB of RAM. The IBM PC used an INTEL 8088 processor. The 8088 processor is a 16 Bit processor that has a 16 Bit internal data bus and an 8 Bit external data bus. The 8 Bit external data bus meant that the PC could use the cheap, readily available old 8 Bit I/O chips, used in the 8 Bit computers. Some other early DOS computer manufacturers used the 8086 processor instead of the 8088. The 8086 is a true 16 Bit processor with a 16 Bit internal data bus and a 16 Bit external data bus. The 8-088 processor illustrated above has only 40 connector legs. Apple and Commodore computers were not based on the 8080 family of processors.


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CPU Development – 80286 based computer IBM's next development was the IBM AT, released in 1983. The AT used a 80286 processor, a true 16 Bit processor with a 16 Bit external data bus and a 24 Bit address bus. The 24 Bit address bus gave the AT access to a maximum of 16 Meg of addressable memory space, they were referred to as 286’s. They were superceded by 386’s (In 1987, the "32 Bit" 80386 processor) and 486’s (486 based machines started off as a 486SX in about 1992 and developed to a 486DX2). In recent times PC’s are referred to as Pentiums. Intel has changed the naming of its processor chips with the introduction of the Pentium processor to stop other manufacturers using the name for their products. If Intel had called this chip the 80586 any other manufacturer could have called its clones of the chip the 80586 also because this is an industry standard naming convention. All of the Intel family of processor chips have been cloned by other manufacturers and when we buy a 486 PC computer we may not be getting an Intel processor chip. When we buy a Pentium PC it must be equipped with an Intel processor.

386 & 486 processors are 32 bit processors, they work with 32 bit numbers rather than 16 bit numbers used by the 1st & 2nd generation PC computers (8088, 8086 & 80286). Pentium is a 64 bit processor & works with 64 bit data. Now there is also an 80686 processor available. This computer naming terminology still refers to the capabilities of the computers CPU. Note that the physical size of the CPU’s has changed little, more computing power has been packed into the same space. Generally processors are unable to be miniaturised any more because the bulk of their size can be attributed to the connections to the chip legs, and sufficient case size to provide cooling. The 486 chip illustrated has 168 connector legs.

In PC’s the CPU is typically connected to a motherboard. Additionally, motherboards contain all the interconnecting circuitry and plug-in points for the remaining electronic circuitry required to operate a modern computer: 

Graphics cards



Sound cards



Memory cards &



Floppy and CD drives.

Also housed within the computer case is a power supply module and a cooling fan (or fans)


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Computer Hardware Hardware refers to the components & assemblies which make-up the computer. Hardware is physical pieces of the computer, from a resistor, to the monitor, keyboard or computer itself.

This hardware includes all the mechanical, electrical, electronic, and magnetic devices within the computer itself (the CPU) and all related peripheral devices (printers, magnetic tape units, magnetic disk drive units, and so on). A hardware fault is repaired by a technician relacing an unserviceable component, & is usually identified by a “hard fail”, eg monitor won’t work, lines on monitor, computer will not run-up, etc.

Computer Software Software plays a major role in computer data processing, without software, the computer could not perform simple addition. It's the software that makes everything happen. It takes a program to make the computer function. You load an operating system into the computer to manage the computer's resources and operations. You give job information to the operating system to tell it what you want the computer to do, copy files, run applications, print data, etc. The operating system receives and processes the job information and executes the programs according to that job information. Software can be defined as all the stored programs & routines needed to fully use the capabilities of a computer.

Computer software is just all 1’s and 0’s. It is the program that when loaded into your computer determines whether you perform calculations or play games. The hardware


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remains the same, it is the software that makes a computer do all the wondrous things. The software load is the instructions the CPU uses to process data. It is like a recipe, the pot and stove is the hardware, but the ingredients and how they are processed determines the outcome, from Pavlova to lambs-fry. It is common for software to be more expensive than the computer system itself. A PC typically costs under $3000. A full suite of MS software and a few games soon overtakes the cost of the computer hardware. In the aircraft industry, Automatic test equipment is commonly used to fault isolate avionics components and circuitry. The cost of purchasing the ATE is far outweighed by the cost of purchasing the software to perform the fault isolation. Aircraft too are loaded with software, from flight management systems computers to flight control computers and navigation computer systems, all are loaded with software which is periodically updated to overcome problems or glitches or to update with more accurate and contemporary data.

Software Storage Mediums Reel to Reel; Floppy Discs: 8 inch, 5¼”, 3½”; Cassette Tape; CD, DVD, etc BIOS is A kind of Flash Memory built onto motherboard. The BIOS software has a number of different roles, but its most important role is to load the operating system. When you turn on your computer and the microprocessor tries to execute its first instruction, it has to get that instruction from somewhere. It cannot get it from the operating system because the operating system is located on a hard disk, and the microprocessor cannot get to it without some instructions that tell it how. The BIOS provides those instructions Flash Memory Card are tiny & have no moving parts. Often used in digital cameras.

Secondary Storage – These forms of memory storage are called secondary storage or auxiliary storage (compared to RAM, ROM, Hard drives, etc. This is memory outside the main body of the computer (CPU) where we store programs & data for future use. When the computer is ready to use these programs and data, they are read into internal storage. Secondary (auxiliary) storage media extends the storage capabilities of the computer system. We need it for two reasons. First, because the computer's internal storage is limited in size, it cannot always hold all the data we need. Second, in secondary storage, data and programs do not disappear when power is turned off. Secondary storage is nonvolatile. This means information is lost only if you, the user, intentionally erase it. Common magnetic storage methods are magnetic disk, tape, and drum.


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Memory All memory (internal storage) sections must contain facilities to store computer data or instructions (that are intelligible to the computer) until these instructions or data are needed in the performance of the computer calculations. Before the stored-program computer can begin to process input data, it is first necessary to store in its memory a sequence of instructions, and tables of constants and other data it will use in its computations. The process by which these instructions and data are read into the computer is called loading. Actually, the first step in loading instructions and data into a computer is to manually place enough instructions into memory using the keyboard or electronically using an operating system (eg DOS), so that these instructions can be used to bring in more instructions as desired. In this manner a few instructions are used to bootstrap more instructions. Some computers make use of an auxiliary (wired) memory (BIOS) that permanently stores the bootstrap program, thereby making manual loading unnecessary.

The memory (internal storage) section of a computer is essentially an electronically operated file cabinet. It has a large number (usually several hundred thousand) of storage locations; each referred to as a storage address or register. Every item of data and program instruction read into the computer during the loading process is stored or filed in a specific storage address and is almost instantly accessible.

Memory Storage Devices For the CPU to control and coordinate all processing activity, it must be able to locate each instruction and data item in storage. Consider the storage as nothing more than a collection of letterboxes. Each letterbox has a unique address and represents a location in memory. Like the mail in your letterbox, the contents of a storage location can change, but the number on your letterbox or memory address always remains the same. In this manner, a particular program instruction or data item that is held in storage can be located by knowing its address. Some computers can address each character of data in memory directly. Others address computer words which contain a group of characters at a single address. Each computer word contains a group of characters at a single address. Some of the more common types of internal storage media used in today's computers are Magnetic core, Magnetic tape, Magnetic disk, and Semiconductor.


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Magnetic Core Storage Although magnetic core storage is no longer as popular as it once was, its concepts are easily understood and apply generally to the more integrated semiconductor and bubble-type memories. Magnetic core storage is made up of tiny doughnut-shaped rings made of ferrite (iron), that are strung on a grid of very thin wires. Since data in computers is stored in binary form, a two-state device is needed to represent the two binary digits (bits), 0 for off and 1 for on. In core storage, each ferrite ring can represent a 0 or 1 bit, depending on its magnetic state. If magnetized in one direction, it represents a 1 bit, and if magnetised in the opposite direction, it represents a 0 bit. These cores are magnetized by sending an electric current through the wires on which the core is strung. It is this direction of current that determines the state of each core.

Core storage is nonvolatile - data is retained even if there is a power failure or breakdown, since the cores store data in the form of magnetic charges rather than electric current.

Magnetic Disc Storage The popularity of disk storage devices is largely because of their direct-access capabilities. Magnetic disks resemble phonograph records, coated with a magnetisable recording material (iron oxide), but their similarities end there. Magnetic disks come in many different sizes and storage capacities. They range from 3 inches to 4 feet in diameter and can store multimillions of bytes. They can be portable (removable), or permanently mounted as hard drives. They can be rigid metal (hard disks) or flexible plastic (floppy disks or diskettes).

Music is stored on a phonograph record in a continuous groove that spirals into the centre of the record. But there are no grooves on a magnetic disk. Instead, data is stored on all disks in a number of invisible concentric circles called tracks. Each track has a designated number beginning with track 000 at the outer edge of the disk. The numbering continues sequentially toward the centre to track 199, or whatever the highest track number is. No track ever touches another.


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Data is written as tiny magnetic bits (or spots) on the disk surface. Eight-bit codes are generally used to represent data. Each code represents a different number, letter, or special character. When data is read from the disk, the data on the disk remains unchanged. When data is written on the disk, it replaces any data previously stored on the same area of the disk. Characters are stored on a single track as strings of magnetized bits (0's and 1's). The 1 bits indicate magnetised spots or ON bits. The 0 bits represent unmagnetised portions of the track or OFF bits. Although tracks get smaller as they near the centre of the disk, each track can hold the same amount of data because data density is greater on tracks near the centre.

Magnetic Tape Storage Another type of storage device is magnetic tape which is similar to the tape used with commercial tape recorders. It is used mainly for secondary storage. It differs from commercial tape in that it is usually wider (ranging from one-half inch to an inch), and it is manufactured to more rigid quality specifications. It is made of a MYLAR base coated with a magnetic oxide that can be magnetized to store data. Magnetic tape comes in a variety of lengths (from 600 to 3,000 feet), and is packaged in one of three ways: open reel, cartridge, or cassette. Large computers use standard open reels, ½ inch wide tape, 2,400 feet in length. Magnetic tape units are categorized by the type of packaging used for the tape. Cartridge tape units can be used on personal computers to provide backup for hard disk.

On magnetic tape each track is set out as indicated by the illustration. 1’s & 0’s are recorded as strings of data (Serial access memory). For data to be retrieved, tape must be cycled through to appropriate position for read/write head to extract data.

Semiconductor Storage (Silicon Chip) Semiconductor memory consists of millions of tiny electronic circuits etched on a silicon chip. Each of these electronic circuits is called a bit cell & can represent a 0 or 1 bit, depending on whether or not current is flowing in that cell. Some of the advantages of semiconductor storage are 

fast internal processing speeds,



high reliability,



low power consumption,



high density (many circuits),



low cost.

Silicon memory is volatile – data is lost when power is removed. Should the power on your computer fail and you have no backup power supply, all the stored data is lost.


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Classifications of Internal Storage We have covered some of the general functions of the CPU, the physical characteristics of memory, and how data is stored in the internal storage section. Now, we will explain yet another way to classify internal (primary or main) storage. This is by the different kinds of memories used within the CPU: 

Read-Only Memory (ROM),



Random-Access Memory (RAM),



Programmable Read-Only Memory (PROM), and



Erasable Programmable Read-Only Memory (EPROM).

Read-only Memory (ROM) In silicon memory devices, usually data is lost when power is removed. Silicon based ROM differs from silicon based RAM. The term ROM is associated with solid state memory devices. The earliest form of read only memory was chiseled stone tablets. Only the computer manufacturer can provide these programs in ROM & once done, they cannot be changed. Consequently, you cannot put any of your own data or programs in ROM.

In most computers, it is useful to have often used instructions, such as those used to bootstrap (initial system load) the computer or other specialised programs, permanently stored inside the computer. Memory that enables us to do this without the programs and data being lost (even when the computer is powered down) is called read-only memory. Many complex functions such as routines to extract square roots, translators for programming languages, and operating systems can be placed in ROM memory. Since these instructions are hard wired (permanent), they can be performed quickly & accurately. Another advantage of ROM is that your computer facility can order programs tailored for its needs and have them permanently installed in ROM by the manufacturer – microprograms or firmware.

Programmable Read-Only Memory (PROM) An alternative to ROM is programmable read only memory (PROM) that can be purchased already programmed by the manufacturer or in a blank state. By using a blank PROM, you can enter any program into the memory. However, once the PROM has been written into, it can never be altered or changed. The main disadvantage of PROM is that if a mistake is made and entered into PROM, it cannot be corrected or erased. Also, a special device is needed to "burn" the program into PROM.


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PROM is a way of allowing a user to tailor a microcode program using a special machine called a PROM programmer. This machine supplies an electrical current to specific cells in the ROM that effectively blows a fuse in them. The process is known as burning the PROM. This means that once programmed, the memory will act as a ROM and will not be corrupted or erased erroneously, and will not erase with loss of power

Erasable Programmable Read-only Memory (EPROM) The erasable programmable read-only memory (EPROM) was developed to overcome the drawback of PROM. PROMs can also be purchased blank from the manufacturer and programmed locally at your locality. Again, this requires special equipment. The big difference with EPROM is that it can be erased if and when the need arises. Data and programs can be retrieved over and over again without destroying the contents of the EPROM. They will stay there quite safely until you want to reprogram it by first erasing the EPROM with a burst of ultra-violet light. This is to your advantage, because if a mistake is made while programming the EPROM, it is not considered fatal. The EPROM can be erased and corrected. Also, it allows you the flexibility to change programs to include improvements or modifications in the future. EPROM more expensive than PROM.

Random Access Memory (RAM) Another kind of memory used inside computers is called random-access memory (RAM) or read/write memory. RAM memory is rather like a blackboard on which you can scribble down notes, read them, and rub them out when you are finished with them. In the computer, RAM is the working memory. Data can be read (retrieved) from or written (stored) into RAM just by giving the computer the address of the location where the data is stored or is to be stored. When the data is no longer needed, you can simply write over it. This allows you to use the storage again for something else.


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RAM is where all the calculating is done by the processor, storing data which it can then randomly access instantly. A CPU will run very slowly reading and writing all calculations direct to a hard drive, as if you were working from a filing cabinet returning each piece of data accesses before accessing another piece. In this respect RAM is like your desk space where you perform all you tasks, then store it all back in the filing cabinet (hard drive) when complete, or when starting another task (program). RAM is the best known form of computer memory & is considered "random access" because you can access any memory cell directly if you know the row & column that intersect at that cell. The opposite of RAM is serial access memory (SAM). SAM stores data as a series of memory cells that can only be accessed sequentially (like a cassette tape). If the data is not in the current location, each memory cell is checked until the needed data is found. SAM works very well for memory buffers, where the data is normally stored in the order in which it will be used (a good example is the texture buffer memory on a video card). RAM data, on the other hand, can be accessed in any order.


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Flight Management System A typical FMS has the capability of automatically controlling the aircraft from just after take-off through to roll out on the runway after landing at the destination. The human pilot must takeover to turn off the runway and taxi to the gate. Not all flights use a flight management system to its fullest capacity, but the autopilot and flight director will be used for some portion of each flight under normal circumstances.

Flight Management Computers Provide a number of advanced features and functions which were not found on earlier autopilot systems. Some of the functions of the FMC’s are: Flight Planning – the entire flight can be programmed into the computer using a cockpit keyboard, cruise altitudes, waypoints, overfly points, ascent and descent points, etc. Performance Management – the system can provide optimum profiles for climb, cruise descent and holding patterns. A minimum cost flight can be flown automatically by using optimum climb settings, cruise settings, etc. Navigation Calculations – the FMC can calculate great circle routes, climb and descents profiles, etc. Auto tune of VOR & DME – FMC can automatically tune radios to the correct and appropriate station frequencies for navigation aids as well as tower approach frequencies. Autothrottle Speed Commands – these are displayed on the EADI as fast/slow indications. The FMC is in effect the master computer which integrates the functions of the inertial navigation unit, flight control computers, thrust management computers, air data computers, navigation sensors and EICAS computers. The Flight Management System (FMS), in conjunction with other interfacing equipment in the aircraft, forms an integrated, full-flight regime control and information system which provides automatic navigation, guidance, map display, and in-flight performance optimization. It reduces cockpit workload by eliminating many routine tasks and computations normally performed by the flight crew. The system operates continuously at all times if properly initialised.


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FMC be coupled to the autopilot, flight director, and auto throttle to provide guidance through integrated commands for controlling roll, pitch and engine thrust and provides course guidance for RNP/RNAV (Radio Navigation Point / Area Navigation) operations. All aircraft systems which are controlled by a computer also run built-in-tests on all associated sensors and processing circuitry to confirm system serviceability, and to electronically report equipment failures to initiate maintenance action. This automatically run BIT greatly increases confidence in system serviceability, and decreases maintenance effort required by performing its own periodic testing, commonly whenever power is applied. System reliability is significantly improved by the combination of BIT and the low failure rate of solid state components. Permits data to be shared with all other avionic systems – without dedicated analogue wiring to interface components In the 1950s and 1960s, aviation electronics, referred to as avionics, were simple stand-alone systems. The navigation, communications, flight controls, and displays consisted of analog systems. Often these systems were composed of multiple boxes, or subsystems, connected to form a single system. Various boxes within a system were connected with point-to-point (analogue) wiring. The signals mainly consisted of analog voltages, synchro-resolver signals, and switch contacts. The location of these boxes within the aircraft was a function of operator need, available space, and the aircraft weight and balance constraints. As more and more systems were added, the cockpits became more crowded, the wiring more complex, and the overall weight of the aircraft increased. By the late 1960s and early 1970s, it became necessary to share information between the various systems to reduce the number of black boxes required by each system. A single sensor, for example that provided heading and rate information, could provide that data to the navigation system, the weapons system, the flight control system, and pilots display system. However, the avionics technology was still basically analog, and while sharing sensors did produce a reduction in the overall number of black boxes, the connecting signals became a “rat's nest” of wires and connectors. Moreover, functions or systems that were added later became an integration nightmare, as additional connections of a particular signal could have potential system impacts. Additionally, as the system used point-to-point wiring, the system that was the source of the signal typically had to be modified to provide the additional hardware to output to the newly added subsystem (additional amplifiers, or output multipliers boxes). As such, inter-system connections had to be kept to the bare minimum. By the late 1970s, with the advent of digital technology, digital computers had made their way into avionics systems. As time and technology progressed, the avionics systems became more digital. And with the advent of the microprocessor, things really took off. The Flight Management Computer typically performs as the data management computer and is responsible for directing the flow of data on the data bus. FMC is master computer which integrates functions of: 

Navigation Computer



Flight Control Computer



Thrust Management Computer



Air Data Computer



Maintenance (EICAS or ECAM) Computer


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Navigation Computer An RNAV computer is referred to by several names: •

RNAV Transceiver

•

Course-line Computer (CLC)

•

Track-Line Computer (TLC)

The function of Area Navigation can also be performed by the Flight Management Computer System (FMCS) as part of the overall aircraft flight management data processing and in large commercial aircraft, and in modern aircraft with digital data bus systems the RNAV function will be incorporated into the FMCS. On smaller and less complex aircraft RNAV can be achieved by incorporation of a simple RNAV computer. Where the position calculations described in the previous slides are reasonably easily performed manually, while flying and with many more things occurring simultaneously it is far more desirable that the navigation calculations be performed automatically and electronically. A simple RNAV computer can perform this function, and only requires inputs from the navigation systems already described in this lesson. The VOR/DME and VORTAC systems are the most common in light aircraft, and this is all the input that most simple RNAV computers would require to perform RNAV functions. With GPS systems becoming more inexpensive and commonplace in light aircraft, modern RNAV computers are likely GPS compatible in addition to the standard VOR/DME and VORTAC inputs.

Inertial Navigation Systems are extremely expensive and are typically only incorporated in large commercial or business jet type aircraft which also incorporate a flight management system and a full suite of navigation systems. An aircraft with an INS would likely not have a separate RNAV computer, as this function would be performed internally by the INS. Computerised navigation systems provide the pilot with a virtually unlimited range of navigational references at his/her fingertips. Older style avionic systems had dedicated indicators and transmit/receive electronics. This cluttered instrument panels, increased the overall weight, and the serviceability was not ideal. (picture of Vulcan Bomber navigators station) By combining the range of navigational aids in one computer and providing a universal display on a Multi-function display (MFD), Horizontal Indicator (HI) or Head-up Guidance System (HGS) the instrument panel is decluttered, reliability is improved and the types of information displayed on the HI can be selected to compliment each other, or the pilot cabin select a single desired source. With additional computerisation of displays and the incorporation of a head-up-display, navigation information can be projected onto the HUD combining glass in the pilots direct field of view, thus displaying typical indications of heading,


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flight director commands, position, etc. Navigation computer systems can provide a continuous display of aircraft track and/or position, compute course and heading for a desired track, display distance and bearing to any selected navigation reference or waypoint. When coupled to a flight control computer (usually through an FMS) a navigation computer can navigate the aircraft at the desired horizontal and vertical track programmed by the pilot, automatically. Area Nav or RNAV permits and aircraft to fly a more direct course to a destination, by plotting its exact position by using the coordination method (intersecting coordinates from VOR stations for example) or inertial navigation or global positioning. RNAV is not generally possible in aircraft without navigation computers capable of referencing several navigation aids simultaneously. Navigating using area nav is far more efficient, in fuel and time savings. With the RNAV computer the pilot effectively moves or off-sets the VOR/DME or VORTAC to any desired location if it is within reception range. This "phantom station" is created by setting the distance and the bearing of the waypoint from a convenient ground station in the appropriate windows of the waypoint selector on the RNAV computer. A series of these "phantom stations" or waypoints make up an RNAV route Navigation computer calculates vertical & lateral guidance including: 

aircraft present position



groundspeed



track angle



desired course



drift angle



wind direction and speed



cross track distance



vertical deviation



vertical speed target



altitude target

 airspeed target. The navigational data base is contained within the navigation computer. It is a world-wide information base comprising of the following data: 

location and frequency of VOR and DME stations



locations of airports



locations of runway thresholds



airway routes and intersection points



standard instrument departure procedures (SIDS)

 standards terminal arrival route procedures (STARs). The database has a capacity of 1.28 megabytes. An information base of this size and complexity can quickly become outdated and is therefore revised every 28 days. The navigation computer uses the database to select and tune the DME and VOR stations which can best provide position fixes. This calculated position fix is used to update the longrange navigation inputs. When beyond the range of VOR and DME transmitters, the navigation computer uses the inertial navigation system or other long-range navigational equipment to perform the computations necessary to keep the aircraft on track. Where two or more systems are


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installed, (e.g. triple IRS), the computer constantly compares the received inputs and averages them out to minimise errors. Instrument landing system (ILS) inputs are used to calculate steering signals during the approach and landing phase of flight.

Area Navigation Area Nav or RNAV permits and aircraft to fly a more direct course to a destination, by plotting its exact position by using the coordination method (intersecting coordinates from VOR stations for example) or inertial navigation or global positioning. RNAV is not generally possible in aircraft without navigation computers capable of referencing several navigation aids simultaneously. Navigating using area nav is far more efficient, in fuel and time savings. With the RNAV computer the pilot effectively moves or off-sets the VOR/DME or VORTAC to any desired location if it is within reception range. This "phantom station" is created by setting the distance & the bearing of the waypoint from a convenient ground station in the appropriate windows of the waypoint selector on the RNAV computer. A series of these "phantom stations" or waypoints make up an RNAV route.


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Thrust Management Computer (TMC) Purpose is to automatically set the proper thrust level for the engines. The output servo motor moves the throttle linkage to set the level of engine power calculated by the TMC. Servos can be electrically powered, or powered by pressurised air, fuel or hydraulics. The system includes sensors on engines which monitor the important engine operating parameters. The monitoring of engine parameters is used to prevent exceeding any engine operating limitation for RPM, EPR, EGT, etc. The autothrottle system can be used to maintain a given climb rate, indicated airspeed, Mach number, or descent rate. The 767 has autoland capabilities and the autothrottle system will automatically close the throttles just prior to landing so that a smooth touchdown can be made. The TMC also provides a minimum speed protection which will maintain a safe margin above stall speed for the particular flight configuration. In order to carry out these functions the thrust management computer database contains the electronic equivalent of the aircraft flight manual. The computer provides output of throttle setting signals for all phases of flight (take-off, go-around, maximum continuous operation, climb and cruise). Outputs of the navigation and thrust management computers can automatically control the aircraft flight path (in all three axis) by operating flight controls and throttles. The computers can also display flight navigation and performance data using a combination of the EFIS and FMS CDU displays.


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Flight Control Computer The pilot moving the control stick in the cockpit generates control inputs. The inputs are electronically sent to the flight control computer which senses what the pilot wants to do bank, dive or climb. The flight control computer sends its signals to the aircraft's flight control surfaces, rudder, ailerons, elevators or stabilizers, which move to perform the desired manoeuvre.

The surfaces selected to perform the manoeuvre are dictated by the software program. During aircraft design and test flying of prototypes, phases of flight are tested within the flight envelope and the software program to manage the flight control system is designed for efficiency and flight safety. Eg selecting a spoiler or aileron to achieve a roll will depend on the rate of roll requested by the pilot (degree of stick deflection), airspeed, AOA and OAT to name but a few. Amount of deflection will also vary greatly with the same stick input depending on the phase of flight. Considering a fighter aircraft, a full surface deflection at low speed will result in a high G turn. The same surface deflection at high speed would likely tear the control surface off, or severely overstress the aircraft. With a CPU managing the flight control surfaces all factors effecting control of flight can be considered before the electrical signal is transmitted to the control surface actuator to deflect the surface the desired amount. It is the flight control computer software load which selects the most efficient method to achieve the attitude change requested by the pilot. Another added benefit is that the flight control surfaces can be easily controlled by other electronic avionic equipment, for example to fly an approach down an ILS localiser and glideslope; autopilot functions of attitude hold, attitude hold, heading hold, etc; coupling the flight control system to the navigation system to fly a pre-programmed course and track. In some aircraft flight control computers can recover from a stall or out-of-control (flat spin) by configuring flight control surfaces such that there is the greatest possibility of the aircraft recovering to controlled flight. The surface positions would be as determined by test pilots during the flight testing of prototypes. In large passenger aircraft the FCS system is programmed to avoid entering a phase of flight where a stall or out of control condition is a possibility, eg stick pushers, stall warning and stall avoidance systems.


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Fly-By-Wire Benefits The Airbus fly-by-wire system offers significant tangible benefits in terms of greater safety, through unconstrained control input freedom within the flight envelope, and protection against exceedance of operating limits, stalling, overspeeding or overstressing the aircraft outside the envelope. To these can be added windshear protection, reduced pilot workload, lower costs and improved aircraft performance. It makes a major contribution to reducing maintenance costs by eliminating much of the complex mechanical system of cables, pulleys and associated gear which need post-maintenance rigging work and checks.

Sidesticks Because the pilot does not have to physically force the control column to deflect the control surfaces with his/her muscular effort, a simple little computer joystick (sidestick) can be incorporated to control the aircraft, with electronic outputs applied to a flight control computer. Replacement of old-generation control columns by modern sidesticks has the benefits of an unobstructed view of the instrument panel and a slide-out working table in front of each pilot. Automatic flight control system elements The basic AFCS consists of the following major elements: Detecting element This consists of attitude gyros, rate gyros, accelerometers, pitot static systems and air temperature probes. They detect the movement of the aircraft about its three flight axes and the rate of that movement. It is considered to be the internal controls or inner loop of the system. Command element This element consists of the pilot’s control panel and manually operated controls, that include pilot’s stick, rudder pedals, navigation systems and radar. It is considered to be the external controls or outer loop of the system. Computer amplifier This is the brain of the system. It computes, amplifies and processes the signals from the detecting and command elements and directs the output element to respond to the pilot’s and/or system’s requirements. Output element This element consists of the units which move the control surfaces of the aircraft in response to the computer demands. These servo units, as they are known, can be electric motors, electromagnetic solenoid valves controlling hydraulic actuators. Fly by wire This system refers to the replacement of mechanical linkages by wires that carry electrical signals from the pilot’s control stick to the servomotors. The movement and forces created by the pilot on his controls are measured by electrical transducers. The signals are amplified and then sent to the various hydraulic actuator units which are directly connected to the control surfaces.


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When an aircraft is able to be mechanically controlled (through electrohydraulic power controlled actuators) and electrically controlled (fly-by-wire), when engaging the AFCS the transition to electric fly-by-wire must be smooth and without violent control surface deflections. The AFCS engagement controlling circuitry will align to the current control surface positions (and will incorporate any current control column or rudder inputs). With no command inputs (stick or pedals) the AFCS will only steer to straight and level flight gradually (non straight and level flight detected by sensors, which all develop error signals) without violent or instantaneous manoeuvring.

Air Data Computers In their earliest form ADC’s where mechanical computers filled with gear trains, cams, aneroid sensors, temperature sensors, etc and were extremely sensitive and delicate components. Parameters were sensed and mechanically converted to electrical signals (potentiometers, etc) for output to other systems or for display. Modern ADC’s are fully electronically computerised. Although the sensing elements are still mechanical type devices, the transducers which convert the mechanical inputs to electrical signals are much more sensitive and do not load down the mechanical sensor, so are also more accurate. Having mostly solid state components ADC’s are very reliable components, rarely suffering unserviceabilities. ADC’s typically receive inputs from Pitot/static system, AOA, OAT (or TOT, Impact Temp, etc). ADC’s also typically receive inputs providing information on flap position, undercarriage position, etc to compensate AOA and pitot/static measurements for position error. The ADC computes air data information from the above inputs to help create a clear picture of flight characteristics. ADC’s constantly perform Selfmonitoring tests, (BIT) to monitor serviceability of sensors & processing circuitry.

All input parameters are measured, converted to electrical signals then digitised for transmission to FMC’s & other avionic systems for display on a multifunction display or HGS. The computer processor performs all these functions, thereby replacing many of the old style analogue instruments such as the ASI, Altimeter, VSI, OAT gauge, AOA gauge. The outputs can be used as inputs to a Flight Control Computer (autopilot & gain scheduling of signals), or thrust management computer (thrust calculations) to automatically maintain an airspeed or altitude, or flight management computer for display on multifunction displays.


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Central Maintenance Computers In their ongoing quest to minimise downtime, many airlines are looking to sophisticated onboard maintenance systems to increase the efficiency with which they handle maintenance requirements. One brand name system is called the “Onboard Maintenance System” (OMS), which is intended to provide rapid fault identification and to thereby enhance maintenance efficiency. The OMS can integrate with ACARS, SATCOM, Wireless LAN and other communications and aircraft systems.

This is a more up-to-date and efficient on board maintenance system, and provides many additional benefits of ECAM and EICAS systems. One of the advantages is advanced notification of fault information. The systems onboard the aircraft can connect to aircraft communication systems and send maintenance information from the aircraft in advance of its arrival at the destination station. That allows maintenance personnel at the destination to begin the troubleshooting process and analysis before the aircraft even gets there. You can also have the maintenance manuals themselves onboard. This is particularly useful where the destination airport is one to which the airline rarely flies, and where the relevant manuals might not be available.


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Maintenance Computers Older commercial airplanes utilise electromechanical system indicators that employ multiple visual and aural cautions and warnings to alert of hazardous conditions such as engine problems or open cabin doors. Most of these systems use an annunciator that provides a master warning light along with an aural indication to alert the crew that a malfunction has occurred and that corrective action may be required. These indicators do not offer the versatility and redundancy available with modern digital technology. New generation aircraft use electronic displays and a full time monitoring system known as EICAS (in Boeing aircraft) or ECAM (in Airbus aircraft). EICAS requires very little monitoring by the crew and promotes quick accurate identification and recording of problems.

The EICAS computer receives inputs from a range of engine and airframe sensors, some of which may be available for display (engine RPM, EGT, etc) and others are monitored purely to provide a caution indication in the event of a failure (cabin door lock, fire warnings, fuel pressure failures, etc). The EICAS computer software program monitors the sensor inputs, and converts some to digital data for transmission to the EICAS display. The computer processors functions in this case would be the high frequency sampling of the input data, at least several times a second, conversion of analogue signals to digital, and monitoring signals to detect out of parameter conditions. By having a computer perform all these functions, the workload on the flight crew is significantly diminished permitting them to focus their attention on actual flying of the aircraft. The Boeing Central Maintenance Computer System CMCS-7000, in conjunction with the Multipurpose Control and Display Unit (MCDU), the Integrated Display System (IDS) and the Multi-Input Cockpit Printer, provides a central point for maintenance functions including: •

flight deck initiation & display of individual subsystem bite & functional tests,

•

selection & display of maintenance messages corresponding to equipment faults & fault history, and

•

control of maintenance related display on the IDS.


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The CMCS provides processing capability and interfaces to other subsystems necessary to perform fault storage, message formatting and bite/functional test of other aircraft subsystems. With two central maintenance computers (CMCs) installed, a failure of one CMC does not affect system operation (Redundancy). Computer printer is located in a convenient position which differs between aircraft types.

The CMC system enables flight crew to make informed decisions based on sound and timely information about the state of the aircraft. The system continuously monitors engines and aircraft systems and generates warnings/cautions and reports that can be customised to individual airline needs. The system provides maintenance manuals, and a wide range of operational flight data (checklists, emergency procedures) which can be viewed on the MATS (maintenance Access Terminal), Portable MATS or printed. The CMC communicates tightly with the integrated display system. If there is a failure which will affect aircraft flight operation it can be displayed on the Engine Indication & Crew Alerting System (EICAS). If the failure relates to a redundant system (not flight critical) it will store the information on the CMC. The pilot can then access that information if he wishes to, and it will also be available to the ground crew when the plane has landed.

The major breakthrough in recent years has been the capability to transfer information to the ground directly through ACARS. Now, instead of having an individual database on every aircraft, the airline can consolidate maintenance information into a centralised database at the ground station. They can also report failures in real-time. As to the future, more “prognostic” capabilities will be incorporated in central maintenance computers. “Theoretically, you could monitor every system to determine how many hours


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they have on them, how many cycles they have gone through and when they are expected to fail. That would allow you to always take corrective action before the failure occurs.

The Control Display Units (CDUs) are mounted on the centre console in the cockpit. They provide control of the CMCs and allow access to CMC fault data. System tests are started from the CDUs, and test results are displayed on the CDU screens.


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TOPIC 5.10 – FIBRE OPTICS History of Fibre Optic Technology People have used light to transmit information for hundreds of years. However, it was not until the 1960s, with the invention of the laser, that widespread interest in optical (light) systems for data communications began. The invention of the laser prompted researchers to study the potential of fibre optics for data communications, sensing, and other applications. Laser systems could send a much larger amount of data than telephone, microwave, and other electrical systems. The first experiment with the laser involved letting the laser beam transmit freely through the air. Researchers also conducted experiments letting the laser beam transmit through different types of waveguides. Glass fibres, gas-filled pipes, and tubes with focusing lenses are examples of optical waveguides. Glass fibres soon became the preferred medium for fibre optic research. Initially, the very large losses in the optical fibres prevented coaxial cables from being replaced. Loss is the decrease in the amount of light reaching the end of the fibre. Early fibres had losses around 1,000 dB/km making them impractical for communications use. In 1969, several scientists concluded that impurities in the fibre material caused the signal loss in optical fibres. The basic fibre material did not prevent the light signal from reaching the end of the fibre. These researchers believed it was possible to reduce the losses in optical fibres by removing the impurities. By removing the impurities, construction of low-loss optical fibres was possible. There are two basic types of optical fibres, multimode fibres and single mode fibres. In 1970, Corning Glass Works made a multimode fibre with losses under 20 dB/km. This same company, in 1972, made a high silica-core multimode optical fibre with 4dB/km minimum attenuation (loss). Currently, multimode fibres can have losses as low as 0.5 dB/km at wavelengths around 1300 nm. Single mode fibres are available with losses lower than 0.25 dB/km at wavelengths around 1500 nm. Developments in semiconductor technology, which provided the necessary light sources and detectors, furthered the development of fibre optics. Conventional light sources, such as lamps or lasers, were not easily used in fibre optic systems. These light sources tended to be too large and required lens systems to launch light into the fibre. In 1971, Bell Laboratories developed a small area light-emitting diode (LED). This light source was suitable for low-loss coupling to optical fibres. Researchers could then perform source-to-fibre jointing easily and repeatedly. Early semiconductor sources had operating lifetimes of only a few hours. However, by 1973, projected lifetimes of lasers advanced from a few hours to greater than 1,000 hours. By 1977, projected lifetimes of lasers advanced to greater than 7,000 hours. By 1979, these devices were available with projected lifetimes of more than 100,000 hours. In addition, researchers also continued to develop new fibre optic parts. The types of new parts developed included low-loss fibres and fibre cables, splices, and connectors. These parts permitted demonstration and research on complete fibre optic systems. Advances in fibre optics have permitted the introduction of fibre optics into present applications. These applications are mostly in the telephone long-haul systems, but are growing to include cable television, computer networks, video systems, and data links. Research should increase system performance and provide solutions to existing problems in conventional applications. The impressive results from early research show there are many advantages offered by fibre optic systems.


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Topic 5.10: Fibre Optics Page 1 of 22


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Definitions of Common Terms Numerical aperture (NA) of the fiber defines which light will be propagated and which will not. NA defines the light-gathering ability of the fiber. Imagine a cone coming from the core. Light entering the core from within this cone will be propagated by total internal reflection. Light entering from outside the cone will not be propagated. A high NA gathers more light, but lowers the bandwidth. A lower NA increases bandwidth. NA has an important consequence. A large NA makes it easier to inject more light into a fiber, while a small NA tends to give the fiber a higher bandwidth. A large NA allows greater modal dispersion by allowing more modes in which light can travel. A smaller NA reduces dispersion by limiting the number of modes Attenuation. Attenuation is loss of power. During transit, light pulses lose some of their energy. Attenuation for a fiber is specified in decibels per kilometer (dB/km). For commercially available fibers, attenuation ranges from approximately 0.5 dB/km for singlemode fibers to 1000 dB/km for large-core plastic fibers.

Optical fibre cables Imagine a long flexible plastic pipe and the inside surface is coated with a perfect mirror. If you look in one end of the pipe & several KM away at the other end, a torch shines into the pipe, you will see the light in the pipe. Because the interior of the pipe is a perfect mirror, the torches light will reflect off the sides (even though the pipe may curve and twist) and you will see it at the other end. If the torch were used to send Morse code you could communicate through the pipe. This is the essence of a fibre optic cable. Making a cable out of a mirrored tube would work, but it would be bulky and it would also be hard to coat the interior of the tube with a perfect mirror. A real fibre optic cable is therefore made out of glass. The glass is incredibly pure so that, even though it is several miles long, light can still make it through (imagine glass so transparent that a window several miles thick still looks clear). The glass is drawn into a very thin strand, with a thickness comparable to that of a human hair. The glass strand is then coated in two layers of plastic (cladding & outer jacket). By coating the glass in plastic, you get the equivalent of a mirror around the glass strand. This mirror creates total internal reflection, just like a perfect mirror coating on the inside of a tube does. Light traveling through the fibre bounces at shallow angles and stays completely within the fibre. To send data through a fibre optic cable, analog signals are translated into digital signals. A laser at one end of the pipe switches on and off to send each bit. Modern fibre systems with a single laser can transmit billions of bits per second – the laser can turn on and off several billions of times per second. The newest systems use multiple lasers with different colours (different frequencies) to fit multiple signals into the same fibre. Modern fibre optic cables can carry a signal quite a distance -- perhaps 100 km. On a long distance line, there is an equipment hut every 70 – 100 km. The hut contains equipment that picks up and retransmits the signal down the next segment at full strength.


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Optical Fibre Communications System A fibre optic data link sends data through fibre optic components to output device. It has the following three basic functions: •

To convert an electrical input signal to an optical signal

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To send the optical signal over an optical fibre

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To convert the optical signal back to an electrical signal

A fibre optic data link consists of three parts—transmitter, optical fibre, and receiver. A fibre optic data link needs a transmitter that can effectively convert an electrical input signal to an optical signal and launch the data-containing light down the optical fibre. A fibre optic data link also needs a receiver that can effectively transform this optical signal back into its original form. The electrical signal provided as data output should exactly match the electrical signal provided as data input.

Transmitter converts the input signal to an optical signal suitable for transmission. It does this by varying the current flow through the light source. The two types of optical sources are light-emitting diodes (LEDs) and laser diodes. The optical source launches the optical signal into the fibre. The optical signal will become progressively weakened and distorted because of scattering, absorption, and dispersion mechanisms in the optical fibre cable. Receiver converts optical signal exiting the fibre back into an electrical signal. An optical detector detects the optical signal. The receiver should amplify and process the optical signal without introducing noise or signal distortion. Noise is any disturbance that obscures or reduces the quality of the signal. An optical detector can be either a semiconductor positiveintrinsic-negative (PIN) diode or an avalanche photodiode (APD). fibre optic data link also includes passive components other than an optical fibre. (splices, couplers, connectors) Passive components used to make fibre connections affect the performance of the data link. These components can also prevent the link from operating. fibre optic components used to make the optical connections include optical splices, connectors, and couplers.


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Basic Structure of an Optical fibre The basic structure of an optical fibre consists of three parts; the core, the cladding, and the coating or buffer. The coating or buffer is a layer of material used to protect an optical fibre from physical damage. Material used for a buffer is a type of plastic. Buffer is elastic in nature & prevents abrasions. Buffer also prevents the optical fibre from scattering losses caused by microbends. Microbends occur when optical fibre is placed on a rough & distorted surface. The core is a cylindrical rod of dielectric material. Dielectric material conducts no electricity. Light propagates mainly along the core of the fibre. The core is generally made of glass. The core is surrounded by a layer of material called the cladding. Even though light will propagate along the fibre core without the layer of cladding material, the cladding does perform some necessary functions. The cladding layer is also made of a dielectric material. The index of refraction of the cladding material is less than that of the core material. The cladding is generally made of glass or plastic. The cladding performs the following functions: •

Reduces loss of light from the core into the surrounding air

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Reduces scattering loss at the surface of the core

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Protects the fibre from absorbing surface contaminants

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Adds mechanical strength

Optical signals are not confined to the core of the fibre. The modes extend partially into the cladding material. Low-order modes penetrate the cladding only slightly, however, high-order modes penetrate further into the cladding material.


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How the cable functions Optical fibre cables propagate a light signal that travels down a fibre glass line by constant refraction off its side walls. The phenomenon of refraction is used to transfer the light from the source to the receiver. Radio waves refract when they leave a media and travel through another media of different density, eg stick dipped in water appears to bend. The angle of refraction depends on the wavelength of the signal, in this case light, being used. The illustration on the slide shows how light transfers down the cable. The wavelengths used in the optical fibre range are from 600 to 1600 nm, infra red, just below the visible light frequency in the electromagnetic spectrum.

Modes of propagation Several light frequencies (or colours if it were visible) can be passed down the same cable simultaneously. Light from each signal source travels down the cable via differing paths, because the difference in frequencies means the angle of refraction is different for each frequency. At the receiver end, the receivers pick out the different frequencies as simply as specific colour in the visible light spectrum can be determined by the human eye – when flashing lights of different colours are visible it is not difficult to focus on a specific colour noting the sequence of flashes – fibre optic receivers function using similar method – they only receive the specific tuned frequency & ignore all others. Low-order or direct mode path uses the cable axis. Middle & high order paths use refraction at different angles. Cables are graded as either graded-index fibre or single-mode fibres.


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Optical fibres are characterised by their structure and by their properties of transmission. Basically, optical fibres are classified into two types – single mode fibres & multimode fibres. As each name implies, optical fibres are classified by the number of modes that propagate along the fibre. The structure of the fibre can permit or restrict modes from propagating in a fibre. Both fibre types are manufactured with the same materials, the basic structural difference is the core size. The core size of single mode fibres is small. The core size (diameter) is typically around 8 to 10 micrometers. The single-mode cable uses only one mode of transfer. It is a much smaller diameter cable used in long distance communication links. Single mode fibres have a lower signal loss and a higher information capacity (bandwidth) than multimode fibres. Single mode fibres are capable of transferring higher amounts of data due to low fibre dispersion. Basically, dispersion is the spreading of light as light propagates along a fibre. As their name implies, graded-index cable (multimode fibres) propagate more than one mode. Number of modes propagated depends on the core size and numerical aperture (NA). Multimode fibres can propagate over 100 modes. The larger core size makes it easier to make fibre connections. During fibre splicing, core-to-core alignment becomes less critical. Another advantage is that multimode fibres permit the use of light-emitting diodes (LEDs). Single mode fibres typically must use laser diodes. LEDs are cheaper, less complex, and last longer. LEDs are preferred for most applications. Multimode fibres also have some disadvantages. As the number of modes increases, the effect of modal dispersion increases. Modal dispersion means that modes arrive at the fibre end at slightly different times. The graded-index cable has a designed refraction index because the velocity of propagation increases away from the centre (light wave travels faster near outer diameter of cable cross section – slower straight through the centre). Although all modes have different paths, they traverse the cable length in about the same time. This type is used for short distances and high data transfer rates.

Losses Attenuation in an optical fibre is caused by absorption, scattering, and bending losses. Attenuation is the loss of optical power as light travels along the fibre. Attenuation reduces the amount of optical power transmitted by the fibre. Attenuation controls the distance an optical signal (pulse) can travel. Once the power of an optical pulse is reduced to a point where the receiver is unable to detect the pulse, an error occurs.

Absorption is a major cause of signal loss in an optical fibre. Absorption is defined as the portion of attenuation resulting from the conversion of optical power into another energy form, such as heat. Absorption in optical fibres is as a result of imperfections in the structure of the optical fibre, & the presence of impurities & contamination.


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Scattering losses are caused by the interaction of light with density fluctuations within a fibre. Density changes are produced when optical fibres are manufactured. During manufacturing, regions of higher and lower molecular density are created (relative to average density of fibre). Light traveling through the fibre interacts with the density areas & light is then partially scattered in all directions.

Bending Losses Bending the fibre also causes attenuation. Bending loss is classified according to the bend radius of curvature: microbend loss or macrobend loss. Microbends are small microscopic bends of the fibre axis that occur mainly when a fibre is cabled. Microbends can be likened to dents in the cladding & core, so the core is no longer smooth & linear – like crushing a co-axial cable reducing its RF handling capabilities. Uneven coating applications and improper cabling procedures increase microbend loss.

Macrobends are bends having a large radius of curvature relative to the fibre diameter. During installation, if fibres are bent too sharply, macrobend losses will occur. Macrobends become a great source of loss when the radius of curvature is less than several centimeters. Light propagating at the inner side of the bend travels a shorter distance than that on the outer side. This condition causes some of the light within the fibre to be converted to high-order modes. These high-order modes are then lost or radiated out of the fibre.


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Dispersion Dispersion spreads the optical pulse as it travels along the fibre. This spreading of the signal pulse reduces the system bandwidth or the information-carrying capacity of the fibre. Dispersion limits how fast information is transferred An error occurs when the receiver is unable to distinguish between input pulses caused by the spreading of each pulse. The effects of attenuation and dispersion increase as the pulse travels the length of the fibre. Dispersion loss is caused when different paths are taken by the various modes of propagation in the cable. A slight variation of the refractive index with wavelength occurs in the fibre. Dispersion loss also occurs when some of the light energy travels in the cladding.

In addition to fibre attenuation and dispersion, other optical fibre properties affect system performance. fibre properties, such as modal noise, pulse broadening, and polarization, can reduce system performance. Modal noise, pulse broadening, and polarization are too complex to discuss as introductory level material. However, you should be aware that attenuation and dispersion are not the only fibre properties that affect performance.

Installation Handling Precautions Primary precautions that need to be emphasised when installing fibre optic systems: Optical fibres or cables should never be bent at a radius of curvature less than a certain value, called the minimum bend radius. Bending an optical fibre or cable at a radius smaller than the minimum bend radius causes additional fibre loss. Extremely sharp bends increase the fibre loss and may lead to fibre breakage •

fibre optic cables should never be pulled tight or fastened over or through sharp corners or cutting edges..

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fibre optic connectors should always be cleaned before mating. Dirt in a fibre optic connection will significantly increase the connection loss and may damage the connector.

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Precautions must be taken so the cable does not become kinked or crushed during installation of the hardware. Extremely sharp kinks or bends increase the fibre loss and may lead to fibre breakage.

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Only trained, authorised personnel should be allowed to install or repair fibre optic systems.


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Optical Fibre Terminations In fibre optic system design, this launching or coupling of optical power from one component to the next is important. fibre optic connections permit the transfer of optical power from one component to another. fibre optic connections also permit fibre optic systems to be more than just point-to-point data communication links. In fact, fibre optic data links are often of a more complex design than point-to-point data links. A system connection may require either a fibre optic splice, connector, or coupler. One type of system connection is a permanent connection made by splicing optical fibres together. A fibre optic splice makes a permanent joint between two fibres or two groups of fibres. There are two types of fibre optic splices--mechanical splices and fusion splices. Even though removal of some mechanical splices is possible, they are intended to be permanent.

Another type of connection that allows for system reconfiguration is a fibre optic connector. fibre optic connectors permit easy coupling and uncoupling of optical fibres. fibre optic connectors sometimes resemble familiar electrical plugs and sockets.

Systems may also divide or combine optical signals between fibres. fibre optic couplers distribute or combine optical signals between fibres. Couplers can distribute an optical signal from a single fibre into several fibres. Couplers may also combine optical signals from several fibres into one fibre. fibre optic connection losses may affect system performance.


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Poor fibre end preparation and poor fibre alignment are the main causes of coupling loss. Another source of coupling loss is differences in optical properties between the connected fibres. If the connected fibres have different optical properties, such as different numerical apertures, core and cladding diameters, and refractive index profiles, then coupling losses may increase. Ideally, optical signals coupled between fibre optic components are transmitted with no loss of light. However, there is always some type of imperfection present at fibre optic connections that causes some loss of light. It is the amount of optical power lost at fibre optic connections that is a concern of system designers. Fibre-to-fibre connection loss is increased by the following sources: •

fibre separation

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Lateral misalignment

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Angular misalignment

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Core and cladding diameter mismatch

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Numerical aperture (NA) mismatch – NA defines the light-gathering ability of the fiber. Imagine a cone coming from the core. Light entering the core from within this cone will be propagated by total internal reflection. Light entering from outside the cone will not be propagated

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Refractive index profile difference – Refractive Index: A property of optical materials that relates to the speed of light in the material versus the speed of light in a vacuum.

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Poor fibre end preparation

Coupling losses are limited by following proper connection procedures & by reducing fibre mismatches between the connected fibres. This is done by procuring only fibres that meet stringent geometrical and optical specifications.


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Fibre End Preparation In fibre-to-fibre connections, poor fibre end preparation can result in coupling loss. A fibreend face must be flat, smooth, & perpendicular to fibre's axis to ensure proper connection. Light is reflected or scattered at connection interface unless connecting fibre end faces are properly prepared. Quality fibre-end preparation is essential for proper system operation. Remove fibre buffer & coating material from end of the optical fibre. Removal of these materials involves the use of mechanical strippers or chemical solvents. After removing the buffer and coating material, the surface of the bare fibre is wiped clean using a wiping tissue. The wiping tissue must be wet with isopropyl alcohol before wiping. The next step involves cleaving the fibre end to produce a smooth, flat fibre-end face. The score-and-break, or scribe-and-break, method is the basic fibre cleaving technique. The score-and-break method consists of lightly scoring (nicking) the outer surface of the fibre then placing it under tension until it breaks. A heavy metal or diamond blade is used to score the fibre. Once scoring is complete, fibre tension is increased until fibre breaks. Fibre is placed under tension either by pulling on the fibre or by bending the fibre over a curved surface. Under constant tension, the score-and-break method for cleaving fibres produces a quality fibre end good enough to use for some splicing techniques. However, additional preparation is necessary to produce reliable low-loss connections when using fibre optic connectors.

Polishing the fibre ends removes most surface imperfections introduced by the fibre cleaving process. fibre polishing begins by inserting the cleaved fibre into the ferrule of a connector assembly (where the fibre will remain after it has been polished – the ferrule is the equivalent of inserting a wire into a socket or pin & crimping it). A ferrule is a fixture, generally a rigid tube, used to hold the stripped end of an optical fibre in a fibre optic connector. An individual fibre is epoxied within the ferrule. The connector with the optical fibre cemented within the ferrule can then be mounted into a special polishing tool for polishing. Fibre inspection and cleanliness are important during each step of fibre polishing. fibre inspection is done visually by the use of a standard microscope to determine if the fibre-end face is flat, concave, or convex.


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Optical Fibre Termination Precautions If involved in terminating fibres, the real dangers are related to the small scraps of glass cleaved off the ends of the fibers being terminated or spliced. These scraps are very dangerous! The cleaved ends are extremely sharp and can easily penetrate your skin. If they get into your eyes, they are very hard to flush out. If swallowed – serious medical consequences will result. Safety glasses are a must! Always follow these rules when working with fiber. 1. Safety glasses must be worn. 2. Dispose of all scraps properly, in a properly marked container. 3. Always work on black pad which makes slivers of glass easier to spot. 4. Do not drop scraps on floor – they’ll stick in carpets or shoes & be carried elsewhere. 5. Do not eat or drink anywhere near the work area.

Fibre Optic Splices A fibre optic splice is a permanent fibre joint whose purpose is to establish an optical connection between two individual optical fibres. Fibre optic splices also permit repair of optical fibres damaged during installation, accident, or stress. Mechanical & fusion splicing are two broad categories that describe fibre splicing techniques. Mechanical splice – mechanical fixtures & materials perform fibre alignment & connection. Fusion splice – localised heat fuses or melts the ends of two optical fibres together. Each splicing technique seeks to optimise splice performance and reduce splice loss. Lowloss fibre splicing results from proper fibre end preparation and alignment. Mechanical splice is a permanent connection made between two optical fibres. Mechanical splices hold the two optical fibres in alignment for an indefinite period of time without movement. The amount of splice loss is stable over time and unaffected by changes in environmental or mechanical conditions. If high splice loss results from assembling some mechanical splices, the splice can be reopened and the fibres realigned. Realignment includes wiping the fibre or ferrule end with a soft wipe, reinserting the fibre or ferrule in a new arrangement, and adding new refractive index material. Once producing an acceptable mechanical splice, splice realignment should be unnecessary because most mechanical splices are environmentally and mechanically stable within their intended application. Typical mechanical splices include glass, plastic, metal, and ceramic tubes; and V-groove devices. Materials that assist mechanical splices in splicing fibres include transparent adhesives and index matching gels. Transparent adhesives are epoxy resins that seal mechanical splices and provide index matching between the connected fibres. The tools to make mechanical splices are cheap, but the splices themselves are expensive.


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Fusion splicing involves using localised heat to melt or fuse the ends of two optical fibres together. The splicing process begins by preparing each fibre end for fusion. Fusion splicing requires that all protective coatings be removed from the ends of each fibre. The fibre is then cleaved using the score-&-break method. Quality of each fibre end is inspected using a microscope. In fusion splicing, splice loss is a direct function of the angles and quality of the two fibre-end faces. The basic fusion splicing apparatus consists of two fixtures on which the fibres are mounted and two electrodes. An inspection microscope assists in the placement of the prepared fibre ends into a fusion-splicing apparatus. The fibres are placed into the apparatus, aligned, and then fused together. Carbon dioxide (CO2) lasers, electric arcs, or gas flames are typically used to heat the fibre ends, causing them to fuse together. The small size of the fusion splice and the development of automated fusion-splicing machines have made electric arc fusion (arc fusion) one of the most popular splicing techniques in commercial applications. By placing the fibre ends between the electrodes, the electric discharge melts or fuses the ends of each fibre. During fusion, the surface tension of molten glass tends to realign the fibres on their outside diameters, changing the initial alignment. When the fusion process is complete, a small core distortion may be present. Small core distortions have negligible effects on light propagating through multimode fibres. Since fusion splicing is inherently permanent, an unacceptable fusion splice requires breakage and refabrication of the splice. In general, fusion splicing takes a longer time to complete than mechanical splicing. Also, yields are typically lower making the total time per successful splice much longer for fusion splicing. Good fusion splicers cost $15,000 to $40,000, but the splices only cost a few dollars each.

Which Splice? If cost is the issue, fusion is expensive equipment and cheap splices, while mechanical is cheap equipment and expensive splices. So if you make a lot of splices (like thousands in an big network) use fusion splices. If you need just a few, use mechanical splices. Fusion splices give very low back reflections and are preferred for single-mode high speed digital networks. However, they don't work too well on multimode splices, so mechanical splices are preferred for MM, unless it is an underwater or aerial application, where the greater reliability of the fusion splice is preferred. Normally, multifibre splices are only installed on ribbon type fibre optic cables. Multifibre splicing techniques can use arc fusion to restore connection, but most splicing techniques use mechanical splicing methods. The most common mechanical splice is the ribbon splice. A ribbon splice uses an etched silicon chip, or grooved substrate, to splice the multiple fibres within a flat ribbon. The spacing between the etched grooves of the silicon chip is equal to the spacing between the fibres in the flat ribbon. Before placing each ribbon on the etched silicon


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chip, each fibre within the ribbon cable is cleaved. All fibres are placed into grooves & held in place with a flat cover. Typically, an index matching gel is used to reduce the splice loss.

Fibre Optic Connectors A fibre optic connector is a demateable device that permits the coupling between two optical fibres or two groups of fibres. Device must allow for repeated fibre disconnect/reconnect without significant loss of light. Fibre optic connectors must maintain fibre alignment during numerous connections. Fibre optic connector coupling loss results from the same loss mechanisms described earlier. Butt-jointed & expanded-beam connectors are two basic types of fibre optic connectors. Buttjointed connectors align & bring prepared ends of two fibres into close contact. End-faces of some butt-jointed connectors touch, but others do not depending upon the connector design. Single fibre butt-jointed and expanded beam connectors normally consist of two plugs and an adapter (Plug-adaptor-plug coupling device). Ferrule connectors use two cylindrical plugs (ferrules), an alignment sleeve, and sometimes axial springs to perform fibre alignment. Precision holes drilled or molded through the centre of each ferrule allow for fibre insertion and alignment. When the fibre ends are inserted, an adhesive (normally epoxy resin) bonds the fibre inside the ferrule (fibre remains inside ferrule – ferrule is like the pin crimped on the end of a wire). The fibre-end faces are polished until they are flush with the end of the ferrule to achieve a low loss fibre connection. Fibre alignment occurs when the ferrules are inserted into the alignment sleeve. The inside diameter of the alignment sleeve aligns the ferrules, which in turn align the fibres. Ferrule connectors lock the ferrules in the alignment sleeve using a threaded outer shell or some other type of coupling mechanism. Fibre alignment depends on an accurate hole through the centre of the ferrule. Normally, ferrule connectors use ceramic or metal ferrules.

Expanded-beam connectors use two lenses to first expand and then refocus the light from the transmitting fibre into the receiving fibre. An expanded-beam connector uses two lenses to expand and then refocus the light from the transmitting fibre into the receiving fibre. Expanded-beam connectors are normally plug-adapter-plug type connections. fibre separation and lateral misalignment are less critical in expanded-beam coupling than in buttjointing. The same amount of fibre separation and lateral misalignment in expanded beam coupling produces a lower coupling loss than in butt-jointing. However, angular misalignment is more critical. The same amount of angular misalignment in expanded-beam coupling


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produces a higher loss than in butt-jointing. Expanded-beam connectors are also much harder to produce. Present applications for expanded-beam connectors include multifibre connections, edge connections for printed circuit boards, and other applications.

Fibre Optic Couplers Some fibre optic data links require more than simple point-to-point connections. These data links may be of a much more complex design that requires multi-port or other types of connections. In many cases these types of systems require fibre optic components that can redistribute (combine or split) optical signals throughout the system. One type of fibre optic component that allows for the redistribution of optical signals is a fibre optic coupler. A fibre optic coupler is a device that can distribute the optical signal (power) from one fibre among two or more fibres. A fibre optic coupler can also combine the optical signal from two or more fibres into single fibre. Fibre optic couplers attenuate the signal much more than a connector or splice because the input signal is divided among output ports.

Fibre optic couplers can be either active or passive devices. The difference between active and passive couplers is that a passive coupler redistributes the optical signal without opticalto-electrical conversion. Active couplers are electronic devices that split or combine the signal electrically and use fibre optic detectors and sources for input and output. An optical splitter is a passive device that splits the optical power carried by a single input fibre into two output fibres. The input optical power is normally split evenly between the two output fibres. This type of optical splitter is known as a Y-coupler. However, an optical splitter may distribute the optical power carried by input power in an uneven manner. An optical splitter may split most of the power from the input fibre to one of the output fibres. Only a small amount of the power is coupled into the secondary output fibre. This type of optical splitter is known as a T-coupler, or an optical tap.

An optical combiner is a passive device that combines the optical power carried by two input fibres into a single output fibre. Issue B: January 2008

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An X coupler combines the functions of the optical splitter and combiner. The X coupler combines and divides the optical power from the two input fibres between the two output fibres. Similar to a breakout on a databus, where several device signals come into a common point for redistribution. Another name for the X coupler is the 2 X 2 coupler.

Star and tree couplers are multiport couplers (passive devices) that have more than two input or two output ports. A star coupler distributes optical power from more than two input ports among several output ports.

A tree coupler splits optical power from one input fibre to more than two output fibres. A tree coupler may also be used to combine the optical power from more than two input fibres into a single output fibre. Star and tree couplers distribute the input power uniformly among the output fibres.

Fibre optic couplers should prevent the transfer of optical power from one input fibre to another input fibre. Directional couplers are fibre optic couplers that prevent this transfer of power between input fibres. Many fibre optic couplers are also symmetrical. A symmetrical coupler transmits the same amount of power through the coupler when the input and output fibres are reversed.


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Optical Fibre System Terminals The amount of optical power launched into an optical fibre depends on the radiance of the optical source. An optical source's radiance, or brightness, is a measure of its optical power launching capability. Radiance is the amount of optical power emitted in a specific direction per unit time by a unit area of emitting surface. For most types of optical sources, only a fraction of the power emitted by the source is launched into the optical fibre. Fibre optic transmitters and receivers are modular components. Fibre optic transmitters and receivers are devices that are generally manufactured with fibre pigtails or fibre optic connectors. A fibre pigtail is a short length of optical fibre (usually 1 meter or less) permanently fixed to the optical source or detector. Manufacturers supply transmitters and receivers with pigtails and connectors because fibre coupling to sources and detectors must be completed during fabrication.

Optical Sources and fibre Optic Transmitters The fibre optic device responsible for converting an electrical analog or digital signal into a corresponding optical signal is a fibre optic transmitter. It converts electrical signals into optical signals and launches the optical signals into an optical fibre. Fibre optic data link performance depends on the amount of optical power (light) launched into the optical fibre. Semiconductor optical sources suitable for fibre optic systems range from inexpensive lightemitting diodes (LEDs) to more expensive semiconductor lasers. Semiconductor LEDs and laser diodes (LDs) are the principal light sources used in fibre optics. The output device from the transmitters generally fall into two categories: optical connectors and optical fibre pigtails. Optical pigtails are attached to the transmitter optical source. The optical source may couple to the output optical connector through an intermediate optical fibre. One end of the optical fibre is attached to the source. The other end terminates in the transmitter optical output connector. The optical source may also couple to the output optical connector without an intermediate optical fibre. The optical source is placed within the transmitter package to launch power directly into the fibre of the mating optical connector. In some cases lenses are used to more efficiently couple light from the source into the mating optical connector. Fibre optic transmitters come in various sizes and shapes. The least complex fibre optic transmitters are typically packaged in transistor outline (TO) cans or hybrid microcircuit modules in dual inline packages (DIPs).


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Optical Detectors and fibre Optic Receivers The optical signals propagating in the fibre become weakened and distorted because of scattering, absorption, and dispersion. The fibre optic device responsible for converting the weakened and distorted optical signal back to an electrical signal is a fibre optic receiver. A fibre optic receiver is an electro-optic device that accepts optical signals from an optical fibre and converts them into electrical signals. A typical fibre optic receiver consists of an optical detector, low-noise amplifier, & other circuitry used to produce the electrical output.

Optical detector converts incoming optical signal into an electrical signal corresponding to the original electrical signal provided to the transmitter. Any variation of the signal at the receiver end is termed distortion – undesirable characteristic. Receiver must replicate original signal. Amplifier then amplifies the electrical signal to a level suitable for further signal processing. Optical detector is a transducer that converts an optical signal into an electrical signal by generating an electrical current proportional to the intensity of incident optical radiation. Optical detectors that meet many of these requirements and are suitable for fibre optic systems are semiconductor photodiodes. The optical fibre is coupled to semiconductor photodiodes similarly to the way optical sources are coupled to optical fibres. Semiconductor detectors are designed so that optical energy (photons) incident on the photodetector produces a current. This current is called a photocurrent. The simplest fibre optic receivers consist of only the optical detector and a load resistor. However, the output signal of these simple receivers is not in a suitable form for most types of interfacing circuitry. To produce a suitable signal, a preamplifier, a post amplifier, and other circuitry are generally included in the receiver. The high-impedance preamplifier is generally used with a large load resistor to improve sensitivity. Fibre optic receivers come in packages similar to those for fibre optic transmitters.


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Advantages of Optical Fibre Cables Advantages of using optical fibre cables in communications are: lighter weight & smaller size: Using optical fibre cables in avionic systems instead of copper wires substantially saves weight and space in aircraft Reduction of cross-talk: No light, therefore no signal, interferes with light on adjacent cables Immunity to electromagnetic interference: EMI does not affect energy at light frequencies Lower signal attenuation: Attenuation figures are approximately 1/100 that of a typical cable or waveguide at the same frequency over a unit length of line Wide bandwidth: bandwidths of 100 MHz up to 1 GHz can be obtained using LEDs and laser light sources. This allows greater signal throughput in a communication system Lower cost: Materials used to construct optical fibres is much less than the cost of copper Safety: Hazards or short circuits and sparks are eliminated Corrosion Resistance: Fibre material is inert & therefore corrosion effects are minimised.

The advantages far outweigh disadvantages, but some disadvantage of using optical fibre cable are: Coupling: from light source to fibre, fibre cable splicing and fibre to detector is very critical exacting joints must be accomplished to avoid displacement losses Special techniques and equipment: must be employed because of the size and nature of the fibre cable material to achieve the proper coupling Ultra-clean environment: must be used when terminating to avoid small particle pollution.


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Aircraft Applications of Optical Fibre During flight aircraft avionics transmit & receive RF signals to/from antennas over coaxial cables. As the density & complexity of onboard avionics increases, the electromagnetic interference (EMI) environment degrades proportionally. The coaxial cables are inherently lossy, limiting the RF signal bandwidth while adding considerable weight. Fibre optics communications links can overcome these limitations. Utilising a form of multiplexing, simultaneous transmission of multiple signals (including RF) can be achieved over a single optical fibre. Relatively small size and weight saving of optical fibre make it ideal for incorporation in avionics where weight & space savings are a premium. With all the advantages of optical fibre it has the capacity & preference to become an integral part of aircraft to carry RF communication between antennas & the cockpit.

Fibre-optic systems consist of transmitters and receivers connected by fibre optic cable. The benefits they offer over traditional copper wire or coaxial cable include the ability to transmit and receive larger amounts of data at higher transmission speeds for greater distances with fewer errors. At the same time, they do not emit radiation or cause any electrical interference and they’re immune to interference from outdoor atmospheric conditions like lightning. And because the basic fibre cables are made of glass, they will not spark if broken, they cannot corrode and they are unaffected by most chemicals. Concerns arising from airplane crashes have caused fibre optics to be incorporated into newer fuel measuring systems. Ever since the crash of TWA Flight 800 was attributed to a spark in one of the fuel tanks, the search has been underway for safer ways to measure fuel tank quantities. In response to this need, optically-powered tank wall processor units for fuel quantity indicating systems have developed. The Boeing 777 has Eleven ARINC 629 data buses and a single optical-fiber data bus route data from aircraft systems through the Airplane Information Management System. Fiber optic technology has been implemented within diverse areas of aircraft vehicle management systems, including propulsion and flight controls. At least four different fiberbased technologies have been demonstrated in the laboratory and some have accumulated flight hours while installed in technology demonstration aircraft. Fiber optic sensing techniques for measuring temperature, position, speed, & flame presence were passively demonstrated on the F404-400 turbofan engine for a NASA program. Hardware and software were developed for optical feedback links in the flight control system of an F/A-18 aircraft. Developments included passive optical sensors and optoelectronics to


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operate the sensors. Sensors with different methods of operation were obtained from different manufacturers and integrated with common optoelectronics. The sensors were the following: Air Data Temperature; Air Data Pressure; Flight Control Position Sensors, stick & rudder position, and Engine Power Lever Control Position (all traditionally monitored by LVDT’s). The integrated system was installed by NASA for flight testing. The most successful sensor approaches were selected for use in a follow-on program in which the sensors will not just be flown on the aircraft and their performance recorded; but, the optical sensors will be used in closing flight control loops (for feedback signals). Development is underway utilising optical fibre as the basis of helicopter AFCS primary flight controls (carrying error signals to the servoactuators, as opposed to just carrying feedback signals as detailed above with respect to the FA-18 AFCS). A triple-redundant fibre optics network is replacing the fly-by-wire primary flight controls on the Apache helicopter. McDonnell Douglas has developed, installed, & tested a number of fiber optic systems on various commercial & military aircraft, it has become evident that problems associated with installation & maintenance of fiber optic systems is perhaps greatest impediment remaining to their incorporation on aircraft. In avionics, fibre-optic-based systems are now proving cost-effective for instrument flight data acquisition systems installed in new aircraft and used in upgrading older aircraft to meet the latest safety requirements. According to the rules of the National Transportation Safety Board (NTSB), transport aircraft are defined as those capable of carrying twenty or more passengers. For operation within the United States, aircraft meeting this definition must provide a certain minimum amount of flight data—and that amount is dependent upon when the aircraft was manufactured. Avionics systems are turning to fibre to help meet these regulatory requirements. For example, aircraft manufactured before flight data acquisition units (FDAUs) became mandatory are required to provide eighteen different measurements including time, pressure, altitude, indicated airspeed, heading, and so on. Aircraft manufactured after FDAUs became mandatory, but before October 1991, must provide an additional four measurements. For aircraft manufactured after October 1991, twelve more measurements are added for a total of 34. Operators of these two groups of aircraft had until Aug 2001 to comply with NTSB rules. But, for aircraft manufactured since August 2000, the requirement has grown to a total of 88 measurements. Clearly, to meet this monitoring requirement requires a lot of sensors and data movement. In both the upgrading of existing systems and in the more stringent requirements of newer systems, fibre optics have proved that they can provide data gathering capabilities that are flexible, affordable, simple and safe.


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TOPIC 5.11 – ELECTRONIC DISPLAYS Light Emitting Diode The Light-Emitting Diode (LED) is one type of optoelectronic device. The light-emitting diode (LED) was developed to replace the fragile, short-life incandescent light bulbs used to indicate on/off conditions on panels. A LED is a diode which, when forward biased, produces visible light. The light may be red, orange, yellow, green, blue, white or ultraviolet, depending upon the material used to make the diode. LEDs emitting a non-visible light in the infra-red part of the radiation spectrum are also available. These LEDs are invaluable for use in detection applications when used in conjunction with infra-red detector components. Figure shows an LED and its schematic symbol. The LED is designated by a standard diode symbol with two arrows pointing away from the cathode. The arrows indicate light leaving the diode. The LED operating voltage is small, about 1.6 volts forward bias and generally about 10 milliamperes. The life expectancy of the LED is very long, over 100,000 hours of operation.

To turn on an LED you must determine the anode and cathode connections. If you look closely at the red plastic base of an LED you will notice a "flat" spot. The wire that comes out beside the flat spot must connect to the "-" side of a power supply, the other wire to the "+" side. LEDs are designed to work at very low voltages (~ 2V) and low currents. They will be damaged if connected to power supplies rated at over 2 volts. LEDs require resistors to limit current when used with power supplies rated at over 2 volts. To avoid damaging your components, always check the suppliers or manufacturers literature for this information.

Peak Wavelength Single Coloured (monochromatic) LEDs The colour of light is the way we perceive its wavelength. The light radiation spectrum is expressed in "nanometres" (nm) and was standardized by the Commission Internationale d'Éclairage (CIE) in 1931. Unlike incandescent lamps that produce light over a wide spectrum (of which visible light is only a small segment), LEDs emit light over only a relatively small part of the radiation spectrum. Peak wavelength is the technical method of defining the colour emitted by the LED (the wavelength of the emitted light) and is measured in "nanometers". Typical figures range from 450nm (blue), through 535nm (green), 585nm (yellow), 620nm (orange), 700nm (red), up to 950nm (infra red). Generally the output is not at one precise wavelength but is distributed over a narrow range, a graph of intensity against wavelength would show a peak at the specified wavelength. The peak wavelength for any LED component is determined by the chemical make-up of the semiconductor substrate, rather than the current or power dissipated. It makes no difference if the LED is in a coloured or clear package.



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Multicolour LEDs / Bi-Coloured LEDs Within the LED epoxy package are two separate reverse parallel semiconductor chips, each producing a different colour. At any one instant of time only one of the LED chips can emit light, which one depends upon the direction of current flowing through the component. Only one series resistor is required, and is calculated using the same formula given above for "Calculation of Series Resistors for LEDs".

Bi-Coloured LEDs can produce a third colour that is a product of mixing together the two primary colours. For example, a red and green Bi-Coloured LED can produce a yellow light. The simplest method to achieve this is to operate the LED from an AC voltage source. This results in each of the primary colour chips operating during their respective half cycles of the alternating flow of current, but the human eye however perceives the rapidly flickering red and green lights as a constant yellow.

Tri-Coloured LEDs Within the LED epoxy package are two separate semiconductor chips that each produce a different colour. A common lead from the two semiconductor chips is connected internally to produce a 3 terminal component as illustrated. Both "common cathode" (illustrated) and "common anode" types are available.

Used simply these components provide a selectable two-coloured light source by switching the voltage between the two semi-conductor chips. Alternatively both semi-conductor chips can be operated simultaneously to mix the two primary colours. Only one series resistor is required providing that both semi-conductor chips are never operated simultaneously. Otherwise it is essential that each chip is protected by its own separate dedicated resistor.





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7-segment LED Display Operation LEDs are used widely as "power on" indicators of current and as displays for pocket calculators, digital voltmeters, frequency counters, etc. For use in calculators and similar devices, LEDs are typically placed together in seven-segment displays. This display uses seven LED segments, or bars (labeled A through G in the figure), which can be lit in different combinations to form any number from "0" through "9."

The schematic, shows a common-anode display. All anodes in a display are internally connected. When a negative voltage is applied to the proper cathodes, a number is formed. For example, if negative voltage is applied to all cathodes except that of LED "E," the number "9" is produced. If the negative voltage is applied to all cathodes except LED "B," the number "6“ is produced.

There are two types of LED 7-segment displays: common cathode (CC) and common anode (CA). The difference between the two displays is the common cathode has all the cathodes of the 7-segments connected directly together and the common anode has all the anodes of the 7-segments connected together. Shown above is a common anode seven segment. As shown above in the CA, all the anode segments are connected together. When working with a CA seven segment display, power must be applied externally to the anode connection that is common to all the segments. Then by applying a ground to a particular segment connection (a-g), the appropriate segment will light up. An additional resistor must be added to the circuit to limit the amount of current flowing thru each LED segment. The above diagram shows the instance when power is applied to the CA connection and segments b & c are grounded causing these two segments to light up. A common cathode seven segment is different from a common anode segment in that the cathodes of all the LEDs are connected together. For the use of this seven segment the common cathode connection must be grounded and power must be applied to appropriate segment in order to illuminate that segment.





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Alphanumeric Led Display Alphanumeric LED displays operate similar to 7-segment. Typically, 16 segments are used in these displays.

Dot Matrix LED Display The more flexible display which is commonly used to produce the full alpha-numeric range is the 35-dot matrix in which LED die are mounted in a 7 x 5 array. With these types of displays, the range of applications increases to a much higher level as compared to the seven segment display range of use.

Alphanumeric LED displays may be an older technology, but its uses are still numerous, even with the wide array of new technologies available for displaying information. Their biggest advantage is cost. For the most part, it is hard to find a cheaper solution for a design that requires displaying small amounts of information. Another very good quality is its durability. The only thing to go wrong is the possibility of a disconnection on connections, because in effect, that's all the diode is, an electrical connection. These displays are generally very simple to program. The logic to decode incoming data for the display is very simple that in fact, a designer can buy a desired decoder fairly cheaply, that is if it doesn't come with the display. Also because of its simplicity, the maintenance is quite low.





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Liquid Crystal Displays Polarization Light is made up of electromagnetic radiation (waves). Light waves can travel in any direction/orientation.

A polarized filter allows only light traveling in one position to pass through. It is made of parallel micro-sized slits that block out all but one position of wave. Polarization is the process of causing light to vibrate in one plane only. Cross polarizing lenses will stop light altogether.

Liquid Crystal The liquid crystal phenomenon was discovered by H. Reinitzer, an Australian, in 1888. Liquid crystal is an organic substance that has both solid crystalline and liquid characteristics within certain temperature ranges. Unlike liquid substances, liquid crystal demonstrated a crystalline structure and related refraction characteristics. Depending on the crystalline state, different refraction's are possible.

In 1968, Williams of RCA Corporation discovered that the way light passes through liquid crystal changes when the liquid is charged with electricity. Five years later, Heilmeyer and his colleagues, also from RCA, applied this property in a display device. Calculators, digital watches, portable word processors, and notebook PCs all use nematic liquid crystals which change their structure with the application of electric voltage.





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Liquid Crystal Reorientation When a molecules with such a characteristic is brought in a sufficiently strong electrical field, they tend to align themselves in the direction of the field. Originally the orientation is almost flat. When an electrical field with direction E is applied (represented in red) there is a force T (represented in green) that tends to align the molecule parallel to the field. When the field is strong enough, the molecule will be almost parallel to the field.

Liquid Crystal Displays A LCD consists of two plates of glass, sealed around the perimeter, with a layer of liquid crystal fluid between them. The liquid crystal layer is a few microns thick. The layer thickness of liquid crystal is approximately 1/10 that of the thickness of an average hair.

Transparent, conductive electrodes are deposited on the inner surfaces of the glass plates. The electrodes define the segments, pixels, or special symbols of the display. Next a thin polymer layer is applied on top of the electrodes. The polymer is etched with channels in order to align the twist orientation of the LC's helix shaped molecules. Finally, polarizing films are laminated to the outer surfaces of the glass plates at 90 degree angles.





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Normally, two polarizing films at 90 degrees should be dark, preventing any transmission of light but due to the ability of LC to rotate polarized light the display appears clear. When AC voltage is passed through the LC, the crystals within this field align so that the polarized light is not twisted. This allows the light to be blocked by the crossed polarizers thus making the activated segment or symbol to appear dark. Liquid crystal displays are commonly known as LCDs. Basically, LCDs operate from a low voltage (typically 3 to 15 V rms), low-frequency (25 to 60 Hz) AC signal and draw very little current. They are often arranged as 7-segment displays for numerical readouts as shown in figure. The AC voltage needed to turn on a segment is applied between the segment and the backplane.

The backplane is common to all segments. The segment and backplane form a capacitor that draws very little current as long as the AC frequency is kept low. It is generally not lower than 25 Hz, because this would produce visible flicker. LCDs draw much less current than LED displays and are widely used in battery-powered devices such as calculators and watches. An LCD does not emit light energy like an LED, and so it requires an external source of light. An LCD segment will turn on when an AC voltage is applied between the segment and the backplane, and will turn off when there is no voltage between the two. Rather than generating an AC signal, it is common practice to produce the required AC voltage by applying out-ofphase square waves to the segment and backplane. This is illustrated in above figure. For one segment. A 40 Hz square wave is applied to the backplane and also to the input of a CMOS 4070 exclusive-OR gate. The other input to the XOR is a CONTROL input that will control whether the segment is ON or OFF.





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When the CONTROL input is LOW, the XOR output will be exactly the same as the 40 Hz square wave, so that the signals applied to the segment and backplane are equal. Since there is no difference in voltage, the segment will be off. When the CONTROL is HIGH, the XOR output will be the INVERSE of the 40 Hz square wave, so that the signal applied to the segment is out of phase with the signal applied to the backplane. As a result, the segment voltage will alternatively be at +5 V and at -5 V relative to the backplane. This AC voltage will turn on the segment.

7-Segment LCD This same idea can be extended to a complete 7-segment LCD as shown above. Here the BCD-to-7-segment decoder/driver supplies the CONTROL signals to each of seven XORs for the seven segments. In general, CMOS devices are used to drive LCDs for two reasons: (1) they require much less power than TTL and are more suited to the battery-operated applications where LCDs are used; (2) the TTL LOW-state voltage is not exactly 0 V and can be as much as 0.4 V. This will produce a DC component of voltage between the segment and backplane that considerably shortens the life of an LCD.





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Reflective LCD Liquid crystal materials emit no light of their own. Small and inexpensive LCDs are often reflective, which means to display anything they must reflect light from external light sources. Look at an LCD watch: The numbers appear where small electrodes charge the liquid crystals and make the layers untwist so that light is not transmitting through the polarized film. Liquid crystal displays (LCDs) have become quite common in watches due to the low electrical power demands of the LCD panel. This panel is composed of two polarizers that transmit light in perpendicular directions, a mirrored surface and a layer of liquid crystal material that is sandwiched between two electrically conducting glass plates. The liquid crystal material used is of the so-called twisted nematic type.

The liquid crystal molecules in all segments of the panel are precisely aligned in the absence of an applied voltage. Therefore, the entire panel appears silvery because light passes through both polarizers, reflects off the mirrored surface, and then passes back through both polarizers.





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If the surfaces of the glass plates that will be in contact with the liquid crystal molecules are rubbed, the liquid crystal molecules orient in the direction of the rubbing. Figure shows that the molecules have been oriented in the direction in which the adjacent polarizer transmits light, and the intervening molecules gradually rotate their relative orientation to accommodate the 90o change from one polarizer to the other. This gives a silvery appearance to the panel.

The initial alignment of the liquid crystal molecules is lost when a voltage is applied to a segment of the panel. That segment will then appear black against a silver background. When a voltage is applied to a segment of the display, the precise alignment of the liquid crystal molecules is lost. This results in the polarized light from the first polarizer not being rotated by the required 90o to align with the second polarizer. The second polarizer blocks the passage of light and causes that segment of the panel to appear black.

Backlit LCD Function in the same manner as normal type LCDs (including reflective type) except they use backlighting. Most computer displays are lit with built-in fluorescent tubes above, beside and sometimes behind the LCD. A white diffusion panel behind the LCD redirects and scatters the light evenly to ensure a uniform display. On its way through filters, liquid crystal layers and electrode layers, a lot of this light is lost -- often more than half!





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Gray Scale LCDs If we carefully control the amount of voltage supplied to a crystal, we can make it untwist only enough to allow some light through. By doing this in very exact, very small increments, LCDs can create a gray scale. Most displays today offer 256 levels of brightness per pixel.

Colour LCDs An LCD that can show colours must have three subpixels with red, green and blue colour filters to create each colour pixel. Through the careful control and variation of the voltage applied, the intensity of each subpixel can range over 256 shades. Combining the subpixels produces a possible palette of 16.8 million colours (256 shades of red x 256 shades of green x 256 shades of blue).

Additive Colour Mixing Additive colour mixing is the mixing of projected beams of coloured light to form other colours. Many find this model hard to understand, simply because it doesn't work as anything you have learned before. But since additive colour mixing is how the eye (and electronic displays) produce colour, it is an important thing to know. Using light to create colours – Shine red, green, and blue (additive primary colours) together to obtain white in the overlap of all three. Conversely, white light can be split into colours (e.g. prisms, rainbows). You get cyan, magenta, and yellow in the other overlaps. These are additive's secondary colours. Black is the absence of light when dealing with additive colours

LCD Subpixels

Red, Green and Blue stripes are arranged in a regular matrix. The gaps between pixels (the shadows) are for the drive circuits and wires. The black lines are significant in that they cannot have their colour changed, yet they take space. Getting as close as possible to the right colours in the right places is what image display is all about. Note how subpixels (magnified) are controlled to obtain colour & clarity

Colour LCDs Colour displays use an enormous number of transistors. For example, a typical laptop computer supports resolutions up to 1,024x768. If we multiply 1,024 columns by 768 rows by 3 subpixels, we get 2,359,296 transistors etched onto the glass! If there is a problem with any of these transistors, it creates a "bad pixel" on the display. Most active matrix displays have a few bad pixels scattered across the screen.





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Cathode Ray Tubes Thermionic Emission – Edison Effect Thomas Edison discovered the principle of thermionic emission as he looked for ways to keep soot from clouding his incandescent light bulb. Edison placed a metal plate inside his bulb along with the normal filament. He left a gap, a space, between the filament and the plate. He then placed a battery in series between the plate and the filament, with the positive side toward the plate and the negative side toward the filament. This circuit is shown. When Edison connected the filament battery and allowed the filament to heat until it glowed, he discovered that the ammeter in the filament-plate circuit had deflected and remained deflected. He reasoned that an electrical current must be flowing in the circuit - EVEN ACROSS THE GAP between the filament and plate. Edison could not explain exactly what was happening. At that time, he probably knew less about what makes up an electric circuit than you do now. Because it did not eliminate the soot problem, he did little with this discovery. However, he did patent the incandescent light bulb and made it available to the scientific community. Let's analyse the circuit in figure below. The heated filament causes electrons to boil from its surface. The battery in the filament-plate circuit places a POSITIVE charge on the plate (because the plate is connected to the positive side of the battery). The electrons (negative charge) that boil from the filament are attracted to the positively charged plate. They continue through the ammeter, the battery, and back to the filament. You can see that electron flow across the space between filament and plate is actually an application of a basic law you already know - UNLIKE CHARGES ATTRACT.





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Remember, Edison's bulb had a vacuum so the filament would glow without burning. Also, the space between the filament and plate was relatively small. The electrons emitted from the filament did not have far to go to reach the plate. Thus, the positive charge on the plate was able to attract the negative electrons. The key to this explanation is that the electrons were floating free of the hot filament. It would have taken hundreds of volts, probably, to move electrons across the space if they had to be forcibly pulled from a cold filament. Such an action would destroy the filament and the flow would cease. The application of thermionic emission that Edison made in causing electrons to flow across the space between the filament and the plate has become known as the EDISON EFFECT. You will remember that metallic conductors contain many free electrons, which at any given instant are not bound to atoms. These free electrons are in continuous motion. The higher the temperature of the conductor, the more agitated are the free electrons, and the faster they move. A temperature can be reached where some of the free electrons become so agitated that they actually escape from the conductor. They "boil" from the conductor's surface. The process is similar to steam leaving the surface of boiling water. Heating a conductor to a temperature sufficiently high causing the conductor to give off electrons is called THERMIONIC EMISSION

Cathode-Ray Tube (CRT) Although you may not be aware of this fact, the CATHODE-RAY TUBE shown in figure is, in all probability, the one tube with which you are most familiar. Before you started your study of electronics, you probably referred to cathode-ray tubes as picture tubes. The cathode-ray tube (CRT) and the picture tube of a television set are one and the same.

Cathode-ray tubes are used in more applications than just television. They can be considered as the heart of the many types of information. Cathode-ray tubes have one function that cannot be duplicated by any other tube or transistor; namely, they have the ability to convert electronic signals to visual displays, such as pictures, radar sweeps, or electronic wave forms. All CRT’s have 3 main elements: an electron gun, a deflection system, and a screen.





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The electron gun provides an electron beam, which is a highly concentrated stream of electrons. The deflection system positions the electron beam on the screen, and the screen displays a small spot of light at the point where the electron beam strikes it. The front of the tube is phosphor-coated and when electrons hit it light will be emitted on the other side (that's the side where you're sitting on).

Electron Gun The cathode of the electron gun in the CRT is required not only to emit electrons, but also to concentrate emitted electrons into a tight beam. In the electron tubes that you have studied, the cathode was cylindrical and emitted electrons in all directions along its entire length. This type of cathode is not suitable for producing a highly concentrated electron-beam. The cathode of the CRT consists of a small diameter nickel cap. The closed end of the cap is coated with emitting material. Because of this type of construction, electrons can only be emitted in one direction.

Notice that the emitted electrons shown in figure are leaving the cathode at different angles. If these electrons were allowed to strike the screen, the whole screen would glow. Since the object of the electron gun is to concentrate the electrons into a tight beam, a special grid must be used. This special grid is in the form of a solid metal cap with a small hole in the centre. The grid is placed over the emitting surface of the cathode and charged negatively in relation to the cathode. The dotted lines represent the direction of cathode emitted electron repulsion, as shown in figure. This repulsion forces emitted electrons to centre and out through hole in grid. Emitted electrons will be forced from the cathode and out through the hole in the grid. Consider an electron leaving from the cathode in an upward direction; Its path will be curved due to electrostatic repulsion. These curving electron paths are due to the negative potential of the grid coupled with the high positive potential of the anode. The potential of the anode attracts electrons out of the cathode-grid area toward the screen. The grid potential may be varied to control the number of electrons allowed to go through the control-grid opening. Since the brightness or intensity of the display depends on the number of electrons that strike the screen, the control grid is used to control the brightness of the CRT.

The proper name, BRIGHTNESS CONTROL, is given to the potentiometer used to vary the potential applied to the control grid. The control grid actually serves as an electron lens. It is this electronic lens that you adjust when you turn up the brightness control on your TV set.





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Notice that the effect of the grid is to focus the electron beam at point P. After passing point P, the electrons start to spread out, or diverge, again. Therefore, it becomes necessary to provide some additional focusing to force the electrons into a tight beam again. This is done by two additional positively-charged electrodes as shown in figure. The first electrode is commonly called the FOCUSING ANODE. Generally, the focusing anode is charged a few hundred volts positive with respect to the cathode. Electrons emitted by the cathode are attracted to the focusing anode. This is the reason that they travel through the small hole in the grid. The second electrode, called the ACCELERATING ANODE, is charged several thousand volts positive in relation to the cathode. Any electrons approaching the focusing anode will feel the larger electrostatic pull of the accelerating anode and will be bent through the opening in the focusing anode and will travel into the area labeled D. You might think that once an electron is in this region, it is simply attracted to the accelerating anode and that is the end of it. This does not happen. Because the accelerating anode is cylindrical in shape, the electrostatic field radiating from it is equal in all directions. Thus, an electron is pulled in all directions at once, forcing the electron to travel down the centre of the tube. Then, the electron is accelerated into the accelerating anode. Once it passes the mid-point (point E), it feels the electrostatic attraction from the front wall of the accelerating anode, which causes it to move faster toward the front. Once the electron reaches point F, equal electrostatic attraction on either side of the opening squeezes it through the small opening in the front of the anode. From there, it is joined by millions of other electrons and travels in a tight beam until it strikes the screen (point S).

The CRT Screen The inside of the large end of a CRT is coated with a fluorescent material that gives off light when struck by electrons. This coating is necessary because the electron beam itself is invisible. The material used to convert the electrons’ energy into visible light is a PHOSPHOR. Many different types of phosphor materials are used to provide different coloured displays and displays that have different lengths of PERSISTENCE (duration of display). The CRT screen can suffer from the effects of secondary emission. In order to reach the screen, electrons from the cathode are accelerated to relatively high velocities. When these electrons strike the screen, they dislodge other electrons from the material of the screen. If these secondary emission electrons are allowed to accumulate, they will form a negativelycharged barrier between the screen and the electron beam, causing a distorted image on the CRT screen. The method used to control secondary emission, which you are already familiar with, i.e., a suppressor grid, is not practical in CRT’s. Instead, a special coating called an AQUADAG COATING is applied to the inside of the tube as shown in figure. This coating is composed of a conductive material, such as graphite, and has the same highpositive potential applied to it that is applied to the accelerating anode. This allows the aquadag to perform two functions. First, since the aquadag coating is positive, it attracts the secondary emitted electrons and removes them. Second, because the aquadag is operated at a high-positive potential and is mounted in front of the accelerating anode, it aids in the acceleration of electrons toward the screen.





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Review Before going on, let's review what you have already learned about CRT operation. Electrons are emitted from a specially constructed cathode and move toward the front of the CRT. The number of electrons that leave the area of the cathode is determined by the potential on the grid. Brightness control. In addition, the grid concentrates the emitted electrons into a beam. The electron beam is focused and accelerated toward the screen by two electrodes: the focusing anode and the acceleration anode. The electron beam strikes the screen and causes a bright spot to appear at the point of impact. Due to phosphor coating. Any electrons released by secondary emission are removed from the tube by the aquadag coating.

Deflection At this point, you have a bright spot in the centre of the CRT screen as shown in figure. Having watched TV, you know that a TV picture consists of more than just a bright spot in the centre of the picture tube. Obviously, something is necessary to produce the picture. That something is called DEFLECTION. For the CRT to work properly, the spot must be moved to various positions on the screen. In your TV set for example, the spot is moved horizontally across the CRT face to form a series of tightly packed lines. As each line is displayed, or traced, the electron beam is moved vertically to trace the next line as shown in figure. This process starts at the top of the tube and ends when the last line is traced at the bottom of the CRT screen. Because the beam is swept very quickly across the CRT and the phosphor continues to glow for a short time after the beam has moved on, you do not see a series of lines, but a continuous picture.

As you should know, there are two ways to move an electron (and thus an electron beam): either with a magnetic or with an electrostatic field. Because of this, there are three possible ways to move or deflect an electron beam in a CRT: magnetically, electromagnetically, and electrostatically. All three ways are used in electronics. In general, though, electrostatic and electromagnetic deflection are used most often. Your TV set, for example, uses electromagnetic deflection, while much of the test equipment in the Navy uses electrostatic deflection.





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Electrostatic Deflection Electrostatic Deflection uses principles you are already familiar with. Namely, opposites attract, and likes repel. Look at figure, view A. Here you see an electron traveling between two charged plates, H1 and H2. As you can see, before the electron reaches the charged plates, called DEFLECTION PLATES, its flight path is toward the centre of the screen. In view B, the electron has reached the area of the deflection plates and is attracted toward the positive plate, H2, while being repelled from the negative plate, H1. As a result, the electron is deflected to the right on the inside of the screen. You, the viewer, will see the spot of light on the left side of the CRT face (remember, you are on the opposite side of the CRT screen). This is shown in view C.

Horizontal Deflection A spot of light on the left-hand side of the CRT screen, however, is no more useful than a spot of light in the centre of the screen. To be useful, this spot will have to be converted to a bright line, called a sweep, across the face of the CRT screen. We will explain the manner in which this is done by using figure.





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In view A, five electrons are emitted in sequence, 1 through 5, by the electron gun. The right deflection plate, H 2, has a large positive potential on it while the left plate, H1 has a large negative potential on it. Thus, when electron 1 reaches the area of the deflection plates, it is attracted to the right plate while being repelled from the left plate. In view B, electron 2 has reached the area of the deflection plates. However, before it arrives, R1 and R2 are adjusted to make the right plate less positive and the left plate less negative. Electron 2 will still be deflected to the right but not as much as electron 1. In view C, electron 3 has reached the area of the deflection plates. Before it gets there, R1 and R2 are adjusted to the mid-point. As a result, both plates have 0 volts applied to them. Electron 3 is not deflected and simply travels to the centre of the CRT screen. In view D, electron 4 has reached the area of the deflection plates. Notice that R1 and R2 have been adjusted to make the right plate negative and the left plate positive. As a result, electron 4 will be deflected to the left. Finally, in view E, the left plate is at its maximum positive value. Electron 5 will be deflected to the extreme left. What you see when facing the CRT is a bright luminous line, as shown in view E. While this description dealt with only five electrons, in reality the horizontal line across a CRT face is composed of millions of electrons. Instead of seeing five bright spots in a line, you will see only a solid bright line. In summary, the horizontal line displayed on a CRT or on the face of a television tube is made by sweeping a stream of electrons rapidly across the face of the CRT. This sweeping action, or scanning, is performed by rapidly varying the voltage potential on the deflection plates as the electron stream passes.





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Vertical Deflection As mentioned earlier, a CRT can be used to graphically and visually plot an electronic signal, such as a sine wave. This is done by using a second set of deflection plates called VERTICAL- DEFLECTION PLATES. In normal usage, the horizontal plates sweep a straight line of electrons across the screen from left to right while the signal to be displayed is applied to the vertical deflection plates. A circuit of this type is shown in the figure below. We will use this figure to explain how a sine wave is displayed. First, however, you need to understand what is happening in view A.

The box on the left of the CRT labeled HORIZONTAL-DEFLECTION CIRCUITS is an electronic circuit that will duplicate the actions of R1 and R2 used earlier in making up a horizontal line. Notice T1; the output of this transformer is applied to the vertical- deflection plates. The signals applied to the vertical plates are 180º out of phase with each other. Thus, when one plate is attracting the electron beam, the other will be repelling the electron beam. Because you are only concerned with what happens inside the CRT, this circuitry will be eliminated and only the CRT and its deflection plates will be shown, as in view B. View C. The sine wave in the centre of the screen is the signal that will be displayed as a result of the two 180º out-of-phase sine waves applied to the vertical-deflection plates. The five spots on the centre sine wave represent the five electrons used to explain horizontal deflection. Only now these electrons will be deflected both vertically and horizontally. Time lines T1 through T5 represent the time when each like-numbered electron reaches the area of the deflection plates. As the electron beam is swept or deflected horizontally, we will not discuss horizontal deflection. Just remember that from T1 to T5, the electron beam will be continuously moved from your left to your right.





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At time 1 (T1), the sine waves applied to both vertical-deflection plates are at their null points, or zero volts. As a result, electron 1 is not vertically deflected and strikes the CRT at its vertical centre. At time 2 (T2), the sine wave applied to the top plate is at its maximum negative value. This repels electron 2 toward the bottom of the CRT. At the same time, the sine wave applied to the bottom plate is at the most positive value, causing electron 2 to be attracted even further toward the bottom of the CRT. Remember, the beam is also being moved to the left. As a result, electron 2 strikes the CRT face to the right of and below electron 1. At time 3 (T3), both sine waves applied to the vertical-deflection plates are again at the null point, or zero volts. Therefore, there is no vertical deflection and electron 3 strikes the CRT face in the centre of the vertical axis. Because the electron beam is still moving horizontally, electron 3 will appear to the right of and above electron 2. At time 4 (T4), the sine wave applied to the top vertical- deflection plate is at its maximum positive value. This attracts electron 4 toward the top deflection plate. The upward deflection of electron 4 is increased by the negative-going sine wave (at time 4) applied to the bottom deflection plate. This negative voltage repels electron 4 upward. Thus, electron 4 strikes the CRT face to the right of and above electron 3. Finally, at time 5 (T5) both input sine waves are again at zero volts. As a result, electron 5 is not deflected vertically, only horizontally. (Remember, the beam is continually moving from right to left.)

Deflection Coils Deflection Coils may also be used in CRTs. These are electromagnets and serve the same purpose as deflection plates.

Magnetic Deflection In order to “trace out” a luminescent display, it is necessary for the spot of light to be deflected about the horizontal and vertical axes, and for this purpose a beam deflection system is provided. A moving electron constitutes an electric current so a magnetic field will exist around it in the same way that a conductor will experience a deflecting force when placed in a magnetic field (motor theory), so an electron beam can be forced to move when subjected to electromagnetic fields acting across the space within the vacuum tube.

Coils are therefore provided around the neck of the tube and are configured by so that fields are produced horizontally (X axis field) and vertically (Y axis field). The deflection coils are





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then connected to the source signal that generates the required display and the electron beam can be deflected left/right and up/down depending on the polarities produced by the coils.

Summary of CRT Operation Here you see both a schematic diagram and a pictorial representation of a CRT. Each element is identified by a circuit number. We will review briefly the function of each element in a CRT and its similarity to elements in conventional tubes. This summary will help you tie together everything you have learned about the CRT.

Heater - heat source for cathode Cathode - source of thermionically emitted electrons outer surface is coated to ensure that electron emission is roughly unidirectional. Control Grid - controls number of electrons that will be fired or emitted Focusing Anode - attracts electrons from grid & focuses into beam Accelerating Anode - accelerates electrons toward front of tube Vertical-Deflection Plates - moves electron beam up and down screen Horizontal-Deflection Plates - moves electron beam horizontally across screen





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In most equipment using CRTs, including TV, electronic signals are supplied to these plates/coils to trace or paint a horizontal line Electromagnetic coils may also be used for horizontal and vertical deflection of the electron beam. Aquadag Coating - positively charged - eliminates effects of secondary emission and aids in acceleration of electrons toward screen Screen - phosphor coated – glows when struck by electrons CRTs operate on 2 principles: •

thermionic emission, and

•

electrostatic/electromagnetic attraction & repulsion





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Safety There are certain safety precautions you should follow when you work with or handle the Cathode Ray Tubes. The glass envelope of a CRT encloses a high vacuum. Because of its large volume and surface area, the force exerted on a CRT by atmospheric pressure is considerable. The total force on a 10-inch CRT may exceed 4,000 pounds. Over 1000 pounds is exerted on the CRT face alone. When a CRT is broken, a large implosion usually occurs. Almost two tons of force hurl glass fragments toward the centre of the tube. At the same time, the electron gun is normally thrown forward inside the tube. The face, because of its size, tends to move very slowly toward the centre of the tube. This presents one of the main hazards of a broken CRT. The electron gun passes through the centre of the tube with considerable force. It continues until it strikes the CRT face. The impact from the electron gun normally breaks the CRT face into many small fragments, which are hurled outward. The face is coated with a chemical coating that is extremely toxic. If you are unfortunate enough to experience an accidental implosion of a CRT and are nicked by one of these fragments, seek immediate medical aid. As you can see, improper handling of a CRT can be very hazardous to your health. The CRT is, in essence, a tiny fragmentation bomb. The major difference between a CRT and a bomb is that a bomb is designed to explode; a CRT is not. As long as you handle a CRT properly, it represents no danger to you. Only when you mishandle it do you risk the danger of being pelted with an electron gun and toxic glass fragments. When handling a CRT, you should take the following precautions: •

Avoid scratching or striking the surface of the CRT.

•

Do not use excessive force when you remove or replace a CRT's deflection yoke or socket.

•

Do not try to remove an electromagnetic-type CRT from its yoke until you have discharged the high voltage from the CRT's anode connector (hole).

•

Never hold the CRT by its neck.

•

Always set the CRT with its face down on a thick piece of felt, rubber, or smooth cloth.

•

Always handle the CRT gently. Rough handling or a sharp blow on the service bench can displace the electrodes within the tube, causing faulty operation.

•

Wear safety glasses and protective gloves.





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Disposal of CRTs One additional handling procedure you should be aware of is how to dispose of a CRT properly. When you replace a CRT, you cannot simply throw the old CRT over the side of the ship, or place it in the nearest dumpster. When thrown over the side of a ship, a CRT will float; if it washes ashore, it is dangerous to persons who may come in contact with it. A CRT thrown in a dumpster represents a hidden booby trap. Therefore, always render the CRT harmless before you dispose of it. This is a fairly simple procedure, as outlined below.

Note: Be sure to wear safety goggles. Place the CRT face down in an empty carton and cover its side and back with protective material. Carefully break off the plastic locating pin from the base by crushing the locating pin with a pair of pliers. Brush the broken plastic from the pin off the CRT base. Look into the hole in the base where the locator pin was. You will see the glass extension of the CRT called the vacuum seal. Grasp the vacuum seal near the end with the pliers and crush it. This may sound a little risky but it isn't. The vacuum seal can be crushed without shattering the tube. Once the seal has been crushed, air will rush into the tube and eliminate the vacuum.





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Coloured CRT’s A colour CRT has 3 electrons guns each of which can direct an electron beam at the screen which is coated with three different types of phosphor material. On being bombarded by electron beams the phosphors illuminate in each of the three primary colours, red, green and blue. The beam from a particular must only be able to strike the small areas or dots of phosphor of one colour, it is not the beam colour that determines the colour displayed on the screen (each gun just fires electrons), it is the colour of the phosphor dot that is illuminated.

Shadow Mask The tricky part is to make sure each electron gun can only hit the phosphor dots for its colour. This is done by arranging three dots of different colours in groups, called triads. Since there are now three electron beams coming from three separate guns, each hits the phosphors from a slightly different angle. A thin sheet called the shadow mask is suspended in front of the phosphors. The shadow mask has one hole for each triad, and is arranged so that each beam can only "see" the phosphor dots for its colour.

The shadow mask is a thin perforated sheet suspended in front of the phosphor dots. There is one hole in the shadow mask for each phosphor triad. Since the three electron beams go through the shadow mask holes from slightly different angles, each beam can only light up the dots for its colour.





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Simultaneous Picture Formation Directly facing the scene is a panel holding enough light sensitive electronic devices, in this case 25 photocells, to be able to cover the whole scene. The light accepted by each photocell is converted into a separate electrical signal, the more intense the light, the larger the amplitude of the signal. These signals are conveyed, each by a separate wire, to a bank of 25 amplifiers. The amplifiers are necessary to increase the signals to a strength adequate for feeding into an array of 25 more lines connected to a distant receiver. The link may involve direct contact, as for closed circuit television, or transmitted communication, as for standard commercial television.

At this point the cost of the system is becoming exorbitant. The provision of 25 amplifiers each adjusted to provide exactly the same characteristics as each other (gain, bandwidth, etc,) is difficult enough, but if transmission is involved, 25 separate frequencies to carry each of the signals will be required, together with an appropriate aerial system and communications arrangement. At the receiver, a similar arrangement of amplifiers for every line in the link will be required. Each of the signals is then fed to a lamp focussed on to its area of the viewing screen. The larger the amplitude of the signal leaving each amplifier, the brighter will shine the lamp it controls, and the stronger the beam of light which the lamp throws onto the viewing screen in the corresponding position. Picture Resolution The inability to resolve fine detail is, of course, principally due to the comparatively small numbers of lamps and photocells used in the system. If the number of both were substantially increased, the resolution of the system would be somewhat improved. A revised letter H could, for example, be made up of 100 areas of illumination, instead of only 25. This would involve a corresponding increase in the number of photocells, lamps, lines and all the other paraphernalia involved in the system. However, in the end, there would be very little improvement in the picture resolution. Clearly, for a TV system to be practical some other technique must be found.





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Sequential Scanning The disadvantage of the primitive simultaneous image reproduction system can be greatly reduced by making use of that peculiar property of the eye, persistence of vision. If signals from the photocells in the camera are not transmitted simultaneously, but are sent one after the other in very rapid succession, persistence of vision in an eye at the receiving end, will create the illusion that the image observed there, is really made up of a large number of simultaneously produced areas of illumination which are free of flicker.

The method which has been devised of transmitting the photocell signals in rapid succession is called Sequential Scanning. It is one of the basic principles upon which all television systems work. Figure shows that every one of the 25 photocells in the transmitter is connected to a fast-moving Scanning Switch, which picks up, one after the other, signals corresponding to the brightness level of the area of the scene which each individual photocell is watching, and feeds it to a common amplifier. Connections to the switch are arranged so that signals are collected from the photocells in sequence, top line first-from left to right, then down to the second line-from left to right, and so on from top to bottom. When the switch reaches photocell number 25, in the bottom right hand corner, it flies back very rapidly to number 1 and begins the cycle all over again. The output from the common amplifier is fed to an aerial and transmitted as a modulated radio frequency signal. At the receiver, the signals picked up are amplified and then connected to another switch, synchronised with that in the transmitter, which in turn is connected to a bank of 25 lamps arranged in the same geometrical pattern as the photocells. The brilliance of lamp number 4 is thus controlled by the signal produced from photocell number 4, and so on. The area of the viewing screen which is illuminated by each lamp, corresponds to the same area of the transmitted scene viewed by the corresponding photocell. One obvious advantage of this system over the more primitive one is that the equipment needed is much reduced. Another is that normal radio communication methods can be applied to the single link between transmitter and receiver. A third is that a large number of channels can be served without serious problems of bandwidth. The key to the system is perfect synchronization of the scanning switch in the transmitter with the lamp selector switch in the receiver. Without it, heavy distortion of the reproduced image will occur.





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Scanning Raster In the picture tube of the TV receiver, the spot is made to move across the screen very rapidly indeed from left to right, and at the same time in a series of horizontal lines from top to bottom. When it reaches the bottom right hand corner of the screen, it is returned very quickly to the top left hand corner, and the scanning cycle is repeated. If this sequence is repeated fast enough, about 50 times per second, the whole screen will have been scanned, and the fluorescence of the top line will have been renewed by a second scan before the light of the first scan has had time to fade in the eye of the viewer. The image presented will be that of a series of parallel lines of light running very close together In almost horizontally across the screen. This presentation, parallel lines of light having no picture content, is known as the Scanning Raster. The Figure below illustrates a raster in which the number of lines has been much reduced, for greater clarity. It should be noted that during the unwanted right to left and bottom to top movement of the spot, called the Flyback Periods, the electron beam is suppressed altogether, and produces no trace.

The scanning raster movement may be seen as similar to the movement of the eye in reading the printed page. Starting at the top left hand corner of the page and scanning fairly slowly left to right, then flying back very quickly to the left hand side to start again on the next line. It will also be obvious that the page scanning rate is much lower than the line scanning rate. Notionally, there will be 50 sets of line scans per second. The raster, in its normal state, produces only a series of parallel lines of light running horizontally across the screen, and presents no picture detail at all. However, the CRT modulator grid voltage can be varied by the picture signal in order to vary, or modulate, the brightness of the raster. It can be taken from the brightest level, called Peak White, to the darkest, called Black, very quickly indeed.





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Interlace Scanning It would appear at this stage that normal TV practice is to cause the beam to scan the screen at no less than 50 times per second, so that flicker of the image is avoided. However, a picture repetition frequency as high as 50 per second would call for a very wide frequency bandwidth for transmission of the video signal, and would therefore greatly restrict the number of channels which could be accommodated within a given frequency band. It would also make receiver design more complex leading to increased receiver cost to the user. An ingenious way has been found of getting around the bandwidth difficulty, it is called Interlaced Scanning. Instead of the target in the camera tube and the screen in the picture tube being scanned in consecutive lines, the beam is first made to scan all the odd numbered lines, in their proper order, followed by all the even lines, down to the penultimate line of the raster.

From there it flies back to the beginning of line 1, and the process is repeated. Thus the picture is comprised of two 'Fields', one for the odd lines and one for the even lines, as shown at Figure. This technique allows the picture frequency to be lowered to 25 per second, reducing the bandwidth problem. The interlacing causes each line, apart from top and bottom, to be formed between two which are fading, it has the effect of cheating the eye into thinking that the picture is being produced at twice the rate. The result is a continuous picture without flicker. The half-lines at top and bottom allow flyback co-ordination without the necessity for special timing elements. The spot is driven horizontally and vertically by two circuits called timebases. The Line Timebase moves the spot from left to right and the Field Timebase, from the top to the

bottom. The video signal itself resets the timebases at the appropriate times to make the spot fly back from right to left and from the bottom to the top. This part of the raster scanning is called, appropriately, the flyback. The spot is turned off, or 'blanked' during flybacks. The Field Timebase scans very much slower than the Line Timebase.

The illustration below shows how the interlaced raster is created. At the beginning of a frame the two timebases are both reset so that the spot is at position (A). Both timebases then start scanning. The video signal triggers line flybacks at the appropriate time until the end of the first field (odd) is reached. Half-way through the last line (B), the field timebase is reset. The spot then flies back to (C), and continues with the remainder of the line, then scans all the even lines in a similar manner. Because the field timebase was reset half way through a line, the even field naturally interlaces between the lines of the odd field.





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When the spot reaches the end of the final line (D), both timebases are reset, and the spot flies back to (A), ready for the next frame. This diagram gives the impression that the scan lines are at an angle. That is because so few lines are shown for clarity. In fact the angle is much less than 1 degree in reality, and is easily corrected by twisting the coils on the tube that deflect the electron beam. The diagram also shows that the visible part of the screen is smaller than the area of the scanned raster. This is because it actually takes several line-scans for the frame timebase to fly back to the beginning (rather than the instantaneous paths shown for clarity on the diagram). The slight 'overscan' on the lines themselves also allows the spot time to stabilise before starting its active scan. The PAL system comprises 625 lines, of which 574 are active. NTSC has 525 lines, of which 485 are active. During the time of inactive lines, it is customary to find non-image data stored in digital form. Examples of this are Teletext (Europe) and VITC timecode. Interlacing then, is a very effective way of reducing the flicker on the picture without increasing the amount (known as 'bandwidth') of transmitted information . The problem is, it can sometimes produce an undesirable vertical 'bounce' on images that have strong horizontal detail. Look at the illustration, which shows a magnified capital F, as it may appear on the screen.





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Aircraft CRT’s In an aircraft CRT, the symbol generators control the picture painted on the screen, controlling the firing of the electron beams at the appropriate coordinates on the screen grid to paint single colour pictures, which are refreshed about 50 times a second.

This is the basis of all the electronic displays, with the CRT providing the visual means to display the information, and the symbol generator controlling the firing of the electron guns as the barrel of the electron gun traces its path across the screen in all those lines, thus producing the visible image. So, for example, when the pilot selects VOR as a heading reference, the symbol generator removes the previous navigation reference display, and has an arrow painted on the compass rose pointing to the selected VOR station. Some aircraft have only one or two CRT’s, while others with a full glass cockpit system will use six or more CRT’s. The electronic instruments that make up a full glass cockpit come in four main types: 

Electronic Attitude Director Indicators (EADI)



Electronic Horizontal Situation Indicators (EHSI)



Engine Indicating and Crew Alerting System (EICAS) or Electronic Centralized Aircraft Monitoring system (ECAM)



Head Up Display (HUD) or Head Up Guidance System (HGS)





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Primary Flight Display (PFD) Also commonly called an Electronic Flight Instrument System (EFIS). A typical EFIS system consists of four interchangeable CRT displays (an EADI and EHSI for the captain and an EADI and EHSI for the first officer), 3 symbol generators, 2 control panels, and 2 Source selection panels. The third (centre) symbol generator is incorporated so that its drive signals can be switched to either the captain or first officers display unit in the event of their primary symbol generator failing.

The symbol generators provide the analogue, discrete and digital interfaces between an aircrafts navigation and sensor systems, and the display units and the control panel. They produce the signals to drive the deflection plates in the display to produce the required symbology, provide power control and system monitoring. The display controller provides the pilot with switches and buttons to select the information sources he/she wishes to have displayed relevant to the phase of flight, eg ILS for approach, VOR/DME for autopilot heading, RADAR display to identify storms and windshear effects, etc. The source select provides a choice between several similar inputs, Radar altitude or Barometric Altitude, INS attitude or Backup attitude source, Inertial heading (true) or magnetic heading, etc





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Multifunction Displays and Digital Data Bus The use of digitally based microprocessor electronics allows several mechanical instruments to be replaced with modern electronic flight instruments displaying the information on one or more multi-function display (MFD’s) or Digital Display Indicators (DDI’s).

The signals sent to the various systems components of the systems are typically linked through a digital data bus. Alternatively on a fully computerised aircraft with a data bus, the information is transmitted to the symbol generators digitally either directly from the sensors (ARINC 429 & 629) or by the Bus Controller (MIL-STD 1553).

EFIS Instrument Layout In older model aircraft the EFIS displays are mounted directly in front of the pilot and normally conform to the standard T layout. Early displays were dedicated to display only EFIS data generated by the symbol generators controlled by FMC computers, or Nav computers. In these aircraft the CRT’s mounted in between the pilots are typically dedicated to providing EICAS or ECAM information. In later model aircraft with data bus technology, the EFIS display is not restricted to the CRT’s directly in front of the pilot, it can be displayed on whichever display it selected. The modern Display systems include the advantages of: •

Large uncluttered displays, 8 x 10 or 10 x 12, enhances crew efficiency and situational awareness

•

Versatile formats allow the displays to be used interchangeably as a PFD, MFD or EICAS display

•

Primary Flight Display includes: attitude, airspeed, vertical speed and navigation plus TCAS resolution advisories

•

Multi Function Display includes: navigation maps, weather radar, TCAS traffic and aircraft maintenance data

Operation can be broken down into two distinct functions, Electronic Flight Instruments System (EFIS) {PFD and MFD} and Engine Indication and Crew Alerting System (EICAS). The EFIS typically contains four large colour displays and two display control panels. 2 termed the PFD’s (primary Flight Displays) and two called the MFD’s (Multi-function displays) The two central CRT displays are commonly aligned to the EICAS system, but all 6 displays are multi-function interchangeable.





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Primary Flight Display (PFD) and Multi-Function Display (MFD) A PFD combines the information provided by an EADI and an EHSI onto one CRT.

The PFD displays attitude, lateral navigation/compass, flight control and primary air data (altitude / airspeed / vertical speed) functions. The PFD receives data bus inputs from Analog-Digital converters interfacing non-digital equipment to the data bus, other avionics systems, Traffic Alert Collision Avoidance System (TCAS) , inertial reference units and air data system. On modern 6 CRT aircraft, the PFD provides the pilot with flight instrument data, and a second Multi-function display adjacent to the PFD provides backup, in case the PFD or EICAS display fails. The MFD can be used to display parameters, but is an important instrument in that it provides redundancy in the event of PFD or EICAS failure. Of course the PFD and MFD are replicated on the co-pilots instrument panel, providing 2 PFD’s, 2 MFD’s and 2 EICAS screens per flight station, totalling 6 CRT’s. The MFD displays lateral navigation/compass, radar, TCAS, flight management (map/summary) and diagnostic information. The MFD also provides a reversion backup for the PFD or the EICAS if that display fails. The MFD receives the same data bus inputs that are applied to the PFD. It also receives input buses from weather radar, EICAS control panel, Maintenance Diagnostic Computer and Flight Management Computer.

Care of Electronic Instrument Displays Electronic displays and instrument panel lighting should wherever possible be turned down from full brightness to prolong life. Screens of electronic instruments should be cleaned carefully to avoid scratching. Typically only side-to-side action is permitted using only a soft lint-free cloth and approved cleaning detergent/agent. Avoid touching display screens with your fingers, it introduces smudges.




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TOPIC 5.12 – ELECTROSTATIC SENSITIVE DEVICES Static Electricity Static electricity is an electrical charge that is at rest, as opposed to electricity in motion or current electricity. Static charges can be generated by either friction or induction. The most common generation of static charge is the friction electricity developed by rubbing together two non-conductive objects. When a person walks across a carpeted or tile floor, a charge builds up in the body due to the friction between the shoes and floor material. The more you generate this friction static the greater the voltage potential develops in the body, you are acting as a capacitor. Everyone's capacitance to hold charges is different. However a sure sign of static presence is hair standing on end or static discharge sparks. Static discharges can be noticed when you touch an object of lower electrical potential such as a door knob, and a bolt of electricity flows from your charged body to the door knob. This flow of electricity is actually a result of the stored static charge that is being rapidly discharged to the lower potential object This type of static electricity build-up is called a triboelectric charge. Another way to generate static charge is through induction, which occurs when an isolated conductive object is brought near another charged object without actually touching it. The field from the charged object can induce charge to flow if the isolated object itself touches ground. This flow of charge is an ESD event. If the isolated object is located very close to ground, the electric field from the induced charge may break down the air and cause an ESD event.

This discharge that can be felt as well as seen, is commonly referred to as an electrostatic discharge, or "ESD". It is not the static charge generated on the body that does the damage rather it is the difference in the two potentials seeking balance that creates an electrostatic discharge. If large enough, the ESD can be felt as well as seen. The ESD event can be felt at the human sensation threshold of 3000 volts. If one feels or sees the static shock, it is a minimum of 3000 volts. The potential static charge that can develop from walking on tile floors is greater than 15,000 volts, while carpeted floors can generate in excess of 30,000 volts.



Topic 5.12 ESD: Page 1 of 18

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Voltage Produced by Friction Substances which produce static electricity readily include glass when rubbed with silk and ebonite when rubbed with fur. In a natural or neutral state, each atom will have a balanced number of protons and electrons. If electrons are removed the material will become charged. If it loses electrons it will be electrically positive, if it gains electrons it will be electrically negative. When two bodies of matter have unequal charges and are near one another an electrical force is exerted between the two. However since they are not in contact their charges cannot equalise. The existence of such an electric force, where current cannot flow, is referred to a static electricity, or an electrostatic force.

One of the easiest ways to produce a static charge is by friction. When two pieces of matter are rubbed together, electrons can be “wiped off” one material onto the other. If materials used are GOOD CONDUCTORS it is quite difficult to obtain a detectable charge on either, since equalising currents can easily flow between the conducting materials. These currents equalise the charges almost as fast as they are created. A static charge is more readily created between nonconducting materials. When a hard rubber rod is rubbed with fur, the rod will accumulate electrons given up by the fur. Since both materials are poor conductors, very little equalising current can flow and an electrostatic charge builds up. When the charge becomes great enough, the current will flow regardless of good conductivity of the materials. These currents will cause visible sparks and produce a crackling sound.




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Induction Induced Static Charges – Simple Induction The illustration shows a conductor in the presence of an electrostatic field emanating from a positively charged source. The resultant separation of positive and negative charges is defined as simple induction. The free or valance electrons are attracted toward the positively charged source. The same effect will result if there is a strong magnetic field in the vicinity.

Upon removal from the field influence the electrons rush back toward the positive ions to neutralise the imbalanced charge, producing an electric current. This current may be sufficient to cause ESD damage. The initial separation of the electrons when the conductor is first introduced to the electrostatic field may also result in ESD damage. If a sensitive integrated circuit is placed in the vicinity of a strong magnetic or electromagnetic field, the IC can be damaged even though no ESD source comes into contact with the IC. The ESD damage results from the imbalance of charge within the IC, induced by an external magnetic or electrostatic field. An electrostatic field can be resident in many non-conductive substances, nylon, wool, plastics, carpet, etc. So simply by placing a nylon garment near an IC can result in some level of damage to the IC. Therefore materials known to be susceptible to holding significant static charges must be isolated from areas where sensitive IC’s or components containing IC’s are stored. This phenomenon is known as Simple Induction.




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Induction Induced Static Charges – Compound Induction If the conductor (Circuit Card) side farthest away from the magnetic or electrostatic field source is grounded, this provides a path for even more electrons to be attracted from earth into the conductor, further increasing the electrical potential resident in the conductor. After disconnecting the ground (moving the circuit card, but not touching any conductive areas – eg, a storeman) a net negative charge remains on the conductor (overabundance of free electrons), still attracted to the positive source. When the field is removed, or the conductor is taken from the field, the charges equalise throughout the conductor (circuit card) resulting in a negative potential across the conductor surface. This sequence is called compound induction.

Compound induction in an electronics workshop, or any area where sensitive electronic equipment is stored can present double jeopardy to static susceptible items. A conductor with an induced potential gradient between its two extreme edges can cause ESD damage to sensitive components when the potentials neutralise. Thus after the first ESD event (simple induction) and removal of the field source, the conductor may be left with a possibly damaging voltage level of the opposite polarity awaiting a second ESD failure upon contact with another neutral source. Induction-caused failures can occur from induced potentials on a sensitive item itself. One example could result from a printed circuit board being placed on a styrofoam cushion. A charge on the cushion would induce a charge on the board. A person touching the conductive area of the board (eg, a repair technician) could bring about an ESD failure, even if the person were grounded because the imbalance of charges in the circuit card (all the electrons near the inducting source) cause a current to flow from earth into the circuit card from the uncharged or earth potential human as soon as the device is touched.

One of the primary ESD control measures is to purge static generative materials from areas where susceptible items are processed. Notorious static generators include common insulative materials such as teflon, acetate, common plastics, polyethylene, styrofoam, wool, silk, and nylon. These materials tend not only to generate high potentials but are likely to retain them for considerable time, i.e. minutes to hours.




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Metal Oxide Semiconductor (MOS) Devices How it works A MOSFET is a field effect transistor, in that the effect of the voltage applied on the gate increases and reduces a field which effects the current flow from the source to the drain. Where a bipolar transistor is simply turned on & off by the base voltage, a FET functions in a state of higher or lower resistance, so is far more efficient as an amplifier. In a MOSFET, the gate is insulated from the channel by a thin film of silicon dioxide. The MOSFET works by enhancing or depleting current flow through the gate. Using an N channel MOSFET in the example, if a negative voltage is applied to the gate, the negative charge repels current carrying electrons from the channel area. This depletion of current carrying electrons decreases the conductivity of the channel (increases the resistance). The greater the negative voltage, the greater the resistance produced in the channel. Sufficient negative voltage can be applied to deplete the channel completely and the drain current falls to zero. With a positive voltage applied, electrons are attracted into the channel, increasing conductivity.

MOSFETs and equipment containing MOSFETs are susceptible to damage from electrostatic discharge and must be handled with care and with strict adherence to electrostatic handling precautions. If exposed to a high static charge an arc will jump through the silicon dioxide insulating layer and either destroy or severely degrade the operation of the MOSFET. Parts susceptible ESD damage are those in any logic family that require small energies to switch states or small changes of voltage in high impedance lines. For example: n-channel metal oxide semiconductor (NMOS), p-channel metal oxide semi-conductor (PMOS), complementary oxide semi-conductor (CMOS), and low power transistor transistor logic (TTL) items. Not only MOS devoices are susceptible to ESD damage, this type of device has been used as the example in this lesson, but all semiconductor devices have some susceptibility to ESD damage.




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ESD Sensitivity As the human body was originally the most common and damaging source of electrostatic discharge, the most common measurement of ESD sensitivity is by Human Body Model (HBM) electrostatic discharge. In this test a charged 100pF capacitor is discharged into the device via a 1500W resistor. The 100pF capacitor simulates charged stored on the average human body, and the resistor simulates the resistance of the human body and skin. The ESD sensitivity of devices is given as an "ESD withstand voltage", which is the maximum test voltage at which the device did not suffer damage. Typical HBM withstand voltages of various device technologies are given in the following table.

As component technology progresses, internal device sizes reduce and become more ESD sensitive. Many modern components are protected by on-chip protection circuits, without which they would be extremely sensitive. In most cases the design goal is to increase the device ESD withstand voltage to 2 kV. In some cases this goal cannot be met for various reasons - there is often a tradeoff between ESD protection and device performance.

Device Sensitivity Classification The Human Body Model (HBM) testing standard includes a classification system for defining the component sensitivity. This classification systems has a number of advantages. It allows easy grouping and comparing of components according to their ESD sensitivity and the classification gives you an indication of the level of ESD protection that is required for the component. In addition to the HBM testing standard there are several others which also produce classes of sensitivity. We will only present the HBM version.




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Identification of ESD Susceptible Equipment Classes of Devices. Any component which can be damaged by 16,000 volts or less is considered sensitive to ESD. These components include microelectronics devices, discrete semiconductors, film resistors, resistor chips, other thick and thin film devices and piezoelectric crystals. The three ESD sensitive classifications are as follows: Class 1: Extremely sensitive Ranges from 0 to 2 kV Class 2: Sensitive Ranges from 2 to 4 kV Class 3: Less sensitive Ranges from 4 to 16 kV

Circuit Cards. ESD sensitive components installed on circuit cards are still susceptible to ESD. For that reason, circuit card assemblies are treated as ESD sensitive. Equipment containing circuit cards with ESD sensitive components, such as computers, receiver/transmitters, digital display units, encoder/decoders, etc., require special treatment to prevent ESD from entering through connector receptacles and damaging sensitive components.




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Types of ESD Damage Early electronic devices and integrated circuits where quite robust (first transistor illustration). Where once an integrated circuit may have contained several transistors, resistors, capacitors, etc, now they contain millions (IC illustration). Miniaturisation of circuits has meant that components have been reduced in size, and have also become extremely sensitive. Years ago only significant signals could be amplified. In recent times devices have become so sensitive that signals that were once too inconsequential to bias a PN junction can now be applied to extremely sensitive transistors and amplified. Where once circuit switching was in the region of hundreds of time a second, circuits now switch thousands of times a second because the responsiveness of semiconductors has increased significantly. All the improvements in semiconductor circuitry has meant that the circuits themselves have become extremely sensitive, and very easy to damage. Where the base of a transistor may be designed to amplify millivolts, if a static charge of several thousand volts is applied, that transistor will be destroyed. Static damage to components can take the form of: Catastrophic failures - occur in two forms, Direct and Latent; and Upset failures - result in gate leakage. Direct catastrophic failures occur when a component is damaged to the point where it is dead now & will never again function. The ESD event may have caused a junction breakdown, metal melt,or oxide failure. The device's circuitry is permanently damaged causing the device fail. This is the easiest type of ESD damage to find since it usually detected during testing. Latent failures occur when ESD weakens or wounds the component to the point where it will still function properly during testing, but over time the wounded component will cause poor system performance and eventually complete system failure. Latent failures due to ESD occur when a component is sufficiently damaged to shorten its operational life. The component may only be marginally damaged by the ESD event and continue to operate for some time. Degradation continues due to the damaged condition of the component and ordinary operational stress. Because latent failures occur after final inspection when the component is fitted to an aircraft, the cost for repair is very high in terms of aircraft downtime and engineering man-hours needlessly consumed. Latent failure damage may then test serviceable at the avionics test bench, where the component will be returned to service nofault-found, primed to create more aircraft downtime and lost man-hours.

An upset failure occurs when an electrostatic discharge has caused a current flow that is not significant enough to cause total failure, but in use may intermittently result in gate leakage causing loss of software or incorrect storage of information. To the user this represents the software glitches and intermittent faults which are so difficult to replicate and isolate, producing the repetitive snags that plague some aircraft. Upset or latent failures may pass testing in an avionics workshop, or onboard Built-In-testing. In other words, static damage may occur that cannot be felt, seen, or detected through normal testing procedures.




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ESD Damage Here are two images showing ESD damage to a C2 MOS capacitor. What appears to be a slight pinhole on the top image (magnified 175 times) shows to be substantial damage to the trace when examined at 4300X. . This damage could have been prevented had the appropriate ESD safe-guards been in place. Major problems arise when components are only partly damaged. Such damage is hard to detect using the usual test methods, but impairs product reliability and safety. Often it leads to latent failures much later causing major financial loss from the cost of repairs and aircraft downtime. The economic necessity for ESD protective measures is clear, particularly as the costs caused by ESD damage are far higher than the capital investment needed to provide an ESD-protected work station. Of the illustrations on the slide, the black & white shot of the burnt circuit track is a failed IC that was rejected as low input resistance (leaky) at a particular input pin. Sectioning in Japan identified the partial short through the silicon from the top as shown by the small well on the track.

On the illustrations the black & white shot of the burnt circuit track is a failed IC that was rejected as low input resistance (leaky) at a particular input pin. Sectioning in Japan identified the partial short through the silicon from the top as shown by the small well on the track. The series of three B&W pictures are continual zoom ins, this transistor was confirmed failed by ESD. You can see where the discharge energy surge has burned through the weakest points in the oxide layer through to the silicon. You will have no way of knowing when you cause ESD damage to a component. And if latent or upset damage has been caused, even testing the component on a workstation may not detect any failure. The only way of being certain a component has not been subjected to ESD is to take all precautions to avoid it.




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ESD Handling Precautions Do NOT touch the pins of any electronic component Do NOT touch the sockets or pins within the plugs of any electronic equipment Discharge any static electricity on yourself by grasping an earthed conductive surface. When removing ESD sensitive equipment from the aircraft, the aircraft must be grounded and power removed Prior to disconnecting the cables from the equipment, touch the metal case of the equipment to equalise any electrostatic potentials Immediately fit conductive covers/caps to disconnected plugs When installing electronic equipment on the aircraft, touch the outer shell of the plug to the outer shell of the equipment mating connector to equalise electrostatic potentials ESD sensitive equipment must not be opened to expose circuit cards anywhere other than at an ESD workstation Place electronic equipment and subassemblies on a grounded anti-static mat whenever maintenance is performed When working with electronic devices (components & cards) – insert the device into the circuit or connector immediately after removing it from the protective carrier or packaging When shipping or moving any electronic components or circuit cards use only approved antistatic containers and bags Do NOT touch the edge connectors of circuit cards Place shorting straps (shunts) across the edge connectors of circuit cards when the cards are being carried or transported At no time during shipment or storage should packaging identified by an ESD symbol be opened unless at an ESD workstation Never store or place circuit cards or electronic components (aircraft avionic components) in the vicinity of electrostatic or strong magnetic fields Work areas where electronic components are often stored and processed must be designed to minimise generation of electrostatic charges: •

Un-carpeted

•

Earthed storage racks

•

Earthed anti-static mats on storage shelves

•

No magnetic or electrostatic fields in the vicinity

An ESD protected work area should address the following: •

Grounded ESD protective work surface

•

ESD safe flooring

•

Personnel grounding

•

Identification of ESD safe workstations

In work areas where particularly sensitive components are handled (Class 0 & 1): •

Removal or control of static generating sources, eg Styrofoam cups, plastics, etc

•

Usage of ESD smocks

•

Installation of air ionizers




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When actually removing and installing electronic components on a circuit card, or fault isolating a circuit card: •

Connect the chassis of all test instruments, soldering iron tips, and your workbench (if metal) to earth

•

Connect yourself to earth with an anti-static wrist strap

•

Keep ESD sensitive components in conductive foam or aluminium foil. This will keep all the pins shorted together so that no dangerous voltages can be developed between any two pins

FAILURE TO OBSERVE THESE PRECAUTIONS COULD RESULT IN SERIOUS DAMAGE TO ELECTRONIC COMPONENTS WITH EITHER IMMEDIATE OR LATENT OPERATION FAILURE OR PERFORMANCE DEGRADATION ESD Damage is undetectable by the human eye. To see ESD damage the IC must first be dismantled, then magnified many times THE ONLY WAY OF BEING CERTAIN A COMPONENT HAS NOT BEEN SUBJECTED TO ESD IS TO TAKE ALL PRECAUTIONS TO AVOID IT

ESD Protective Packaging Circuit cards and components must be packaged in ESD protective packaging prior to leaving the ESD workstation. Static shielding bags which have a static dissipative inner layer and a conductive outer layer are used for this purpose. Since the days of Michael Faraday, it has been well known that field induced effects can be prevented by enclosure in a highly conductive material, thereby creating an electrostatic shield or Faraday cage. The inside layer of the bag is designed to prevent static build up due to the movement of the component inside the bag. Another layer is made from metal, and is referred to as a Faraday Cage. This metal layer prevents damage to the bag's contents due to Induction Charging. The degree of shielding is a function of the conductivity of the material from which the shield is constructed— the higher the conductivity, the better the shielding.

The other layers of the bag provide strength and a moisture barrier. They must be noncorrosive and must zip lock or heat seal closed. All static shielding bags are identified with an ESD caution sticker over the closed seals, so that broken stickers will become indicators of opened bags. Cushion wrap (bubble wrap) used around circuit cards must also be made of static dissipative material. Circuit cards may be packaged in reusable ESD fast pack containers. At the equipment level, conductive connector receptacle dust caps are used to prevent ESD from entering the equipment through the connector receptacle and damaging sensitive components. In cases where conductive dust caps are unavailable, a conductive grid tape may be used to cover connector receptacles. The shorting of connecting leads or pins of devices by means of wire, spring clips, metal foil or by inserting the leads or pins into a conductive foam or material. For PCBs and card modules having edge connectors, specially formed strips called “shunts” are placed over connectors to keep them at the same potential. For complete electronic assemblies, covers or caps made from a conductive material are placed over the connectors.




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Anti Static Bags – Pink Poly Pink Poly anti-static bags are made from amine-free polyethylene and have approx 2 to 6 mil thickness. Polyethylene resin is impregnated with an anti-static substance which renders them quasi-conductive. The additive allows the material to remain transparent and from the colour of the material the bags are usually referred to as “pink poly”. Pink Poly anti-static bags are recommended for packaging components with Class III sensitivity levels

They have a shelf life of three years, are not damaged by wrinkling or creasing and are heat sealable. They are available with an antistatic zipper allowing frequent opening and closing without reducing the bags antistatic properties. Cushion-pack bags are made from “pink poly” foam material - These Cushioned antistatic bags are manufactured with two layers of laminated 4 mil antistatic pink polyethylene and one layer of antistatic cushioning. The single most important characteristic of a static protective packaging material is its electrical resistance. The less the resistance around the container, the more static protection is provided. Antistatic plastics do not provide the electrostatic shielding required by the extremely fast response of static sensitive devices, this is achieved by using metallised bags. Anti Static Bags – Metallic These bags are manufactured with a transparent layered laminate that provides optimum static shielding with maximum visibility. Transparent metallised static shield bags are designed for packaging products that are very sensitive to low-level static discharges (class 1 sensitivity). The bag's inner layer prevents static from generating inside the bag. It is bonded to a metallised polyester layer which provides static shielding of the contents from external sources. The abrasion resistant exterior helps the bag maintain its static shielding capabilities even under rugged handling and reuse.

These top-of-the-line static shielding bags are transparent for maximum visibility. Their layered lamination prevents static generation inside and outside of the bag while the metallised layer offers optimum static shielding. The multitrack, antistatic zipper is easily opened & closed. Allows bag to be reused many times without loss of the antistatic properties




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Grid Tape Relate how stripping standard cello-tape from a dispenser generates a very high static charge. When surfaces are brought into close contact and then separated, some electrons from one surface will attach themselves to the other surface. This results in an excess of electrons on one surface (a negative charge) and a deficit of electrons (a positive charge) on the other surface. Video footage around of tape coming off dispenser then held near an electrostatic sensing device, registers very high. Cello-tape is a high risk source of static electricity. Grid Tape is anti-static tape, for use in ESD sensitive areas.

ESD Conductive Grid Tape is a beige polypropylene, three layer tape with a conductive grid middle layer. The grid pattern & ESD awareness symbols help alert recipients that packages contain ESD sensitive components and should be handled in an ESD safe work area. Applications for conductive shielding grid tape: For applications requiring EMI shielding Use in areas where the generation of static electricity is of concern Using grounded dispenser, voltage generated unrolling will effectively be reduced to zero For Securing (bundling) anti-static tubes containing IC’s Sealing ESD bags and other ESD packaging / containers Use with ESD symbols for ESD awareness Attach ESD paperwork to bags or product holding notes, etc in antistatic workstations Covers external plugs, holes or connector pins on electronic chassis (black boxes, etc.) during transportation or storage




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Conductive Transit Trays Used for transporting ESD sensitive components in an ESD workshop. Reduces the likelihood of building up static charge during transport around the workshop when all other ESD measures are in place (heel grounders, ESD smocks, ESD safe flooring, etc). These trays can be designed for return and reuse with or without an antistatic lid

Tote Boxes The primary use for tote boxes is for the in-plant transport of devices and PC boards to and from the controlled static-safe work stations. A tote box is an antistatic conductive box with a lid. As in the case of bags, the box must not only prevent triboelectrically generated charge, but to be effective, it must also shield the components from external static fields. The theory of shielding effectiveness of bags (more conductive – more efficient shield) can be extended to cover any closed container, including a tote box. May also be used for storage of small discrete ESD sensitive items at an ESD workbench. Tote boxes with lids provide greater protection than just a conductive tray alone.

Static Protection in DIP Tubes Perhaps the most common carrier used for shipment, storage and automatic handling, is the storage rail or tube designed for holding dual-in-line packages (DlPs). These tubes are fabricated from a variety of materials including aluminum, plain plastic, carbon loaded plastic, and antistatic plastic. Although these tubes are rarely "capped" on the ends, they may, for the purposes of static shielding, be considered as a closed container because of the relatively small size of the end openings. Therefore, the theory developed for protective bags can be applied to the DIP tubes as well.

Antistatic Shipping Materials There are an enormous amount of products now available with antistatic or ESD reduction/elimination incorporated. Just a few more examples are:

Shipping Boxes The black static protected shipping box is an effective means for shipping expensive circuit boards and devices. These shippers are designed with a conductive layer buried in the corrugated fiberboard to help dissipate static surfaces. The interior has a special static dissipative foam cushioning on the top and bottom to help protect against shock and vibration during transportation




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Antistatic Clamshells Used for packaging circuit boards as a replacement of antistatic bags and foam. A more cost effective solution on several fronts including material costs, assembly time, storage space, reduced number of inventory items

People are Prime Sources of ESD Electrostatic charges generated by rubbing or separating materials are readily transmitted to a person's conductive sweat layer causing that person to be charged. When a charged person handles or comes in close proximity to an ESD sensitive component, that component can be damaged by a direct discharge when it is touched or by subjecting it to an electrostatic field.

Grounding Strap A "proper" grounding wrist strap has a resistance built into it, so static electricity will discharge slowly and constantly, but a real current (like when you touch the wrong wire with your screwdriver) meets a brick wall. The wrist strap must have a resistance of 1 Meg-ohm to prevent the worker from electric shock. These straps are comfortable, adjustable, and you can even snap the cord off of the wrist band when you have to leave the work-bench for a minute. The worker must wear the wrist strap directly on his/her skin (not on top of clothing), and check the conductivity of the wrist strap at least once a day to make sure the cable is not broken.

Antistatic Gloves Ideal for handling delicate sensitive parts, films, electronic instruments, circuit boards and components. Assembly and repair work in electronics, telecommunications, precision instrumentation and optics, etc. Photographic film laboratory processing, video film handling and motion picture industry. Electrostatic painting and petrochemical industry

Finger Cots Ideal for ESD-Safe areas, these powder free pink latex finger cots offer a 3 mil thickness




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ESD Safe Smocks To minimise static build-up on normal clothing, and to provide a conductive path for charges in normal clothing to dissipate to earth (when connected to earth)

Heel Strap Grounders As for ESD safe smocks. Provides a conductive path to earth so any charge built up when walking will be dissipated to earth with each step. ESD safe shoes are also available, and ESD safe over shoes, that resemble large gumboots.

ESD Safe Work Envelopes Replace plastic envelopes which can develop very high static charges. Used to protect maintenance instructions and certification paperwork utilised in ESD safe work areas. Most of these devices would only be used realistically in an ESD safe workshop. You will often be expected to wear wrist straps though if you are employed working a great deal with avionics components.




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Workshop Antistatic Devices The best ESD controls are not only the ones that protect sensitive components and equipment but are at hand, readily available, and easily maintained. For these reasons carpets and tile floors should not be overlooked as sources for electrostatic control. Existing carpet or tile floors can be easily included into an ESD control program. An ESD safe workstation protects your sensitive products by providing pathways to safely drain damaging static charge to ground. A wrist strap and coil cord remove static from the person as it is generated. Parts placed on a static dissipative mat decay their static charge to ground at a safe rate. Both the mat and coil cord are connected to ground by the Common Ground Cord System.




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ESD Protection Requirements Electronic components are classified into three groups depending on ESD sensitivity. Each group has different handling requirements. In facilities where numerous types of electronic assemblies and circuit cards are repaired, it is not always evident to which ESD sensitivity group a particular assembly belongs. Consequently, most repair facilities use standard ESD protection procedures based on the most susceptible device they expect to repair. The ESD workstation is an essential part of ESD protection and is the only safe location to repair, package, or handle ESD sensitive components or circuit cards. The purpose of the workstation is to keep potential differences below the level that can damage ESD sensitive components. This is accomplished in several ways. The bench top, floor mat, and a personnel wrist strap are electrically connected together through resistors to ground. In addition, the floor mat, bench top, chair, component containers, and all other materials in the area are made from static dissipative material. No static generators such as plain plastic wrap, styrofoam, plastic coffee cups, etc., are allowed in the area. Humid air helps to dissipate electrostatic charges by keeping surfaces moist and increasing surface conductivity. The workstation and surrounding area should be kept between 40 and 60 percent relative humidity for this purpose. Ionized air generators which produce both positive and negative ions may be used at the ESD workstation to dissipate any static charge. Personnel are often required to wear static dissipative smocks and should avoid wearing synthetic clothing under the smock. ESD workstations must be periodically monitored to ensure all components are functional. The action of air ionizers is to add very nearly equal numbers of positive and negative ions to the air in order to provide charge carriers that increase the electrical conductivity of the air. If only a small percentage of the air molecules are ionized (one millionth to 100 millionth of 1%), the time for static discharge is reduced from hours to seconds The theory of operation for one simple type of ioniser is as follows A very high voltage (~5-20 kV) voltage is applied to a set of sharp points within the ioniser and, an intense electric field is established in the very near vicinity (~100 mm.) of the points. This field accelerates free electrons to a sufficiently high energy to allow them to ionize molecules that they collide with. When the voltage on the point is positive, positive ions are repelled into the environment and when the point is negative, negative ions are delivered. Corona ionizers are simpler and therefore are cheaper to manufacture. They utilise a step-up transformer to create the high voltage for ion generation. Because the AC type ionizer produces the positive and negative ions in sequence from the same emitter, these ions are separated in time by half of the period of the AC power line (½ the frequency) (i.e. 1/100 or 1/120 sec.). This means that the waves of positive and negative ions are rather close to each other, making loss through recombination a large factor. AC ionizers typically utilise fast airflow velocity to minimize recombination. This is not always desirable in a clean room environment.




B1-5a Digital Techniques I SR

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