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5.4 DATA BUSES

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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 relatedsubject.

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Traditional Example STUDENTS FILE

Students 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 ARINC429.

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ABOUT ARINC 429

It defines how avionics equipment should talkwith each other.

Defines the electrical and data characteristics andprotocol.

A 32 bits serial bus with data flow in onedirection. If two directions are required, then twosets 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 data

9-10 SDI Source Destination Information

11-28 Data

29-31 SSM Sign Status Matrix

32

Parity bit - sum of all logic is ODD 10

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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 Redundant777 Primary Flight Computer

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Time Division Multiplex

Linear BusMultiple Transmitter Access2 Mbps Data Rate

Current Mode Coupling(Present implementation)17

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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 blackboxes 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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ARINC 629 MESSAGE STRUCTURE

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Message

Max 31 word string /

gap 4 bits

Each label

20bits/label-12/ext-4/3 syncro/1 parity

Each data word

20bits/ 16 data/ 3 syn/ 1 parity

Each word string

Label + 256 datawords

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MESSAGE STRUCTURE

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