EASA PART-66 MODULE 5.4 : DATA BUSES

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5.4 DATA BUSES

www.part66.blogspot.com

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BITS, BYTES,WORD

• The traditional data hierarchy are:– Bit – either 0 or 1– Bytes – a collection of bits to make a character– Word – a collection of bytes to have a meaningful

word.– Field – a collection of words– A record – a collections of field on a related

subject.

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Traditional Example – STUDENTS FILE

• Student’s File – consist of 100 records of student.

• A record – for example – information on a student.

• Fields – example students name or address or telephone number e.g Abraham Maslow

• Word – the name of a student e.g Abraham• Bytes – e.g A is represented by 10000001• Bit – either 0 or 1

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ARINC 429 DATA WORD

• In computing, word is the natural unit of data used by a particular processor design.

• It is basically a fixed sized group of bits that are handled as a unit by the instruction

• The number of bits in a word (the word size, word width, or word length) is determined by the computer architecture.

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INTRODUCTION TO DATA BUSES

• Data buses are used to transmit data from one component to another.

• The buses can be electrical or fibre optic.• The specification for data transmission is

drawn by ARINC.• The standard used in aircrafts is the ARINC

429.

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ABOUT ARINC 429

• It defines how avionics equipment should talk with each other.

• Defines the electrical and data characteristics and protocol.

• A 32 bits serial bus with data flow in one direction. If two directions are required, then two sets of buses are required. (max 20 receivers)

• The signal uses three level -10v, 0 and +10v. (measured between 2 wires)

• High speed 100k and low speed of 12-14k/sec (cannot be mixed)

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ARINC 429 DATA WORD

• Five Basic Parts– Label (e.g True air speed, TAT)– Source /Destination Identifier (SDI)– Data Field (bit 11-28)– Sign Status Matrix (SSM) (e.g +, - , North) – Parity Bit ( Odd parity)

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DATA TRANSMISSION

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ARINC DATA WORD

1- 8 Label – information about the data9-10 SDI – Source Destination Information11-28 – Data 29-31 – SSM Sign Status Matrix32 – Parity bit - sum of all logic is ODD

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DATA FORMAT

• It is using Bipolar return to zero (RZ) modulation.– The data in 3 states, hi, null, lo (1, null, 0)– A 4 null period separate the words.

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DATA BUS

• Operates on one of the two speeds– High - 100kbits/sec– Low - 12.0kbits/sec

• Self clocking – the receiver receives the null between the bits to generate its internal clock.

• Self synchronous – the 4 bits null is recognized and used to generate word synchronisation.

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VOLTAGE LEVEL

• The voltage used is +10 , 0 and -10 volts.• +10volts signifies logic 1 and (-10v) is logic 0

(obsolute voltage +- 5 )

• The second half of the bit is always null.• The shield is grounded at both end of he wire.

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INSTALLATION

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1977 => Boeing began to work on “DATAC”

project

1977 - 85 => DATAC Emerged as ARINC 629

1989 => ARINC 629 was adopted by AEEC

1990 => ARINC 629 was first implemented

in BOEING-777

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Triple-Triple Redundant 777 Primary Flight Computer

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Time Division Multiplex Linear Bus Multiple Transmitter Access 2 Mbps Data Rate Current Mode Coupling

(Present implementation)

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ARINC 629

• ARINC 629 introduced in May 1995• Used on B777, A330 andA340 .• Multiple-source, multiple sink system• Each terminal can transmit receive data• ARINC 629 can handle total of 120 terminals• It supports a data rate of 2 Mbps

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ARINC 629 SYSTEM - COMPONENTS

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ARINC 629 DATA BUS CABLE

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ARINC 629

BUS INSTALLATION• Most of the wires are in

electronics compartment. This to enable the black boxes to talk to each other.

BUS COUPLER• The coupler is used to

connect the LRU to the data bus.

• A maximum of 120 couplers can be connected.

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429 V 629 BUSES

• A pair if wire per link• One way flow• Max 100kb• Receiver max 20

• Shared buses – max 120• Two ways• Max 2 mega• Receiver – max 120

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ARINC 629 MESSAGE STRUCTURE

Message

Max 31 word string /gap 4 bits

Each label20bits/label-12/ext-4/3 syncro/1 parity

Each data word20bits/ 16 data/ 3 syn/ 1 parity

Each word stringLabel + 256 datawords

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MESSAGE STRUCTURE