Topic 6 Digital Technique Data buses

Malaysian Institute of Aviation Technology
DIGITAL TECH (MECH)
AKD 21102
CHAPTER 6
DATA BUSES
Revision 00 Issue 01 Module 5.4

LEARNING OUTCOME
• To understand the operation of data
buses in aircraft systems, including
knowledge of ARINC and other
specifications

DATA BUSES
• A wire or group of wires used to move
data to/from components and to/from
other computer
• The data can be in the form of electrical
pulses or light signals (fiber optics)

ARINC
• Aeronautical Radio INCorporated (ARINC) was
established in 1929
• Leading provider of the transport communications and
systems and engineering solutions for 5 major industries
i.e. aviation, airport, defence, government and
transportation.
• Primary activity is to produce specifications and reports
for purpose of :
– Indicating to manufacturers the group opinion concerning
requisites of new equipment
– To channel new equipment design in direction which will result in
maximum standardization

ARINC
• Examples of ARINC specifications are:
 ARINC 573 : Digital flight data recording
 ARINC 561 : Inertial navigation systems
 ARINC 429 : Transfer of digital data
between avionics components
 ARINC 629 : Improvement of ARINC 429

ARINC 429
• Is a specification which define how avionic
equipment and systems should communicate to
each other
• Consist of pairs of twisted wires with shielding
• Point to point bus and uses a unidirectional data
bus standard (TX and RX are on separate ports)
• Also known as the Mark 33 Digital Information
Transfer System (DITS)

ARINC 429
• Messages are transmitted at either 12.5 kbit/s or
100 kbit/s to other system element that are
monitoring the bus messages
• Always transmitting either 32 bit data words or a
NULL state

ARINC 429:
Electrical Characteristics
• Self clocking and self synchronizing system
called ‘bipolar return to zero’
• Transmitting in binary bit form where bit
‘1’represent high voltage (+5V) and bit ‘0’
represent low voltage (-5V) level for half of clock
cycle then each pulse return to zero in the
middle of the clock pulse

ARINC 429:
Electrical Characteristics

ARINC 429 :
Protocol
• Only one transmitter on a wire pair
• Always transmit 32 bit word or a NULL state
• At least 1 receiver on wire pair and up to 20
receivers only
• Number of receiver depend on line length and
the number of receivers connected to the bus

Example of ARINC 429 Application in Boeing 747

ARINC 429 :
Word Format
• Word format is a 32 bits with 5 basic parts:
1) Label
2) Source/Destination Identifier (SDI)
3) Data field
4) Sign Status Matrix (SSM)
5) Parity bit (P)

ARINC 429 - Word Format

• LABEL - The first 8 bits of the word (octal coded).
Identifies the information contained in the data e.g.
airspeed, total air temp.
LABEL PARAMETER
230 True Airspeed
231 Total Air Temperature
233 Static Air Temperature
323 Flight Path Acceleration
330 Body Yaw Rate
Example of label

• SOURCE/DESTINATION IDENTIFIER
(SDI)
- used to identify the source/ destination of
a word e. g. Which of a number of
installations the word is coming from or
needs to be directed into.

Malaysian Malaysian Institute Institute of of Aviation Aviation Technology
Technology
• DATA FIELD - contains specific data related to
the LABEL. For binary word contained in bits
11- 28 and binary coded decimal (BCD) in bits
11- 29. Any bit not used are filled with logic 0's
(pad bits)
• SIGN STATUS MATRIX - bits 29,30,31 for
binary word and 30,31 for BCD. Identify
characteristics of the word e.g. east/west, +ve/-
ve and its status e.g. No computed data, failure
warning, functional test or normal operation.

• SSM code for identity characteristic of
the word for BCD data

Malaysian Malaysian Institute Institute of of Aviation Aviation Technology
Technology
• SSM code for status of word for BCD data

• PARITY - Uses odd parity e.g. Total
number of logic '1' in the word must be an
odd number. If not parity bit is set to 1.
- Uses to check an error e.g. If receiving signal
doesn't contain odd number 1, fault signal will
be generated.

ARINC 429 :
Transmission Order
• The least significant bit (LSB) of each byte
except the label is transmitted ahead of the data
in each case. The order as follows:
8,7,6,5,4,3,2,1,9,10,11,12,13,14,….32

ARINC 629
• Additional ARINC standard that is being developed and
is used in new fleet of BOEING 777 aircraft
• Uses a high speed bidirectional bus with the capability of
either periodic or aperiodic transmission
• Access to the bus is controlled by a sophisticated
protocol involving wait period, quiet period and other
rules
• Data bus is unshielded twisted pair of wires, bonded and
terminated at both ends.
• Data is send at the rate of 2 Mbps

Topic 6 Digital Technique Data buses

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