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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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