Module 5: Digital Techniques and
Electronic Instrument Systems
5.4 Data Buses
What is a bus?
Bus: A collection of wires
through which data is
transmitted from one part of a
computer (or one computer)
PC: Connects e.g. CPU, DVD-
ROM, RAM, PCIe card etc.
Mobile device: Connects CPU,
GPU, WiFi controller, etc.
Aircraft: Data highway which
links one LRU (Line
Replaceable Unit) to another.
Address and Data Bus
All busses consist of two
Address bus: To which
device connected to the bus,
the data should go?
Data bus: The actual data to
However, the address and the
data bus can be incorporated
in one, by transmitting a
single data word which
contains the address
information. (This is the case
in the aircraft busses).
Serial vs. Parallel bus
Each bit of the data word
is transferred via a
Requires a lot of wiring.
Each bit of the data word
is transferred via the
Examples: PCIe, USB,
ARINC, I2C, …
Serial vs. Parallel bus
should have been
faster than serial.
Parallel busses suffer
from clock skew. (i.e.
a bit can reach the
destination before or
after other bits: lack
Require more wiring.
synchronized as fast
as the serial busses.
(lower data rate).
Parallel buses are rare today.
Most busses architectures are
Communication between Components
Single source – single sink: One LRU
communicates with a single LRU.
Single source – multiple sink: One LRU
communicates with multiple LRUs at the same time.
Multiple source – multiple sink: Multiple computers
communicate with multiple LRUs at the same time..
Simplex: A data bus can transmit only in one
direction (Any LRU can only transmit or receive data
at any time).
Half – duplex: Can transmit in both directions, but
not at the same time (LRUs can take turns
transmitting or receiving).
Full duplex (Duplex): All LRUs can send and
receive data at the same time.
Other bus characteristics
Width: How many bits can be transmitted at the
e.g. a 16-bit bus can transmit 16 bits simultaneously.
Clock speed (in MHz): How often the bus can
transmit data. Faster clock speed means faster bus.
All these characteristics form the “bus architecture
and protocol”. Civil aircraft busses are defined and
standardized by ARINC.
ARINC: Aeronautical Radio
A company that develops and
operates aviation systems and
ARINC, ACARS (datalink between
aircraft and ground), LRU
Develops also solutions for
defense, networks, security, …
Founded in 1929.
Most common technique: Parity Check
How it works?
An extra bit is used called parity – bit in every data word.
We can use 2 kinds of parity that a bus can use:
Odd parity: Parity bit should have such a value that the total number of “1s” is odd.
Even parity: Parity bit should have such a value that the total number of “1s” is even.
The transmitter sends the data (along with the parity bit) to the receiver.
The receiver counts the “1s” and if the number does not agree with the Parity,
error is detected and the word is sent again.
It is the simplest form of error detection. It is also used in PCs for data
transfer between RAM or HD and CPU.
Binary Encoding Formats
Binary Encoding: How to
represent “1” and “0” in a bus?
3 basic techniques are used in
aircraft data buses:
Bipolar Return to Zero (BPRZ)
Manchester II Non Return to
The clock is embedded in the
The receiver LRU does not need
a clock to decode the data.
Bipolar Return to Zero (BPRZ)
“1” is a positive voltage and return to Null at half bit
“0” is a negative voltage and return to Null at half bit
The return to Null is the way the receiver identifies
every single bit.
Self-clocking: No clock is needed in the receiver.
“1” is positive voltage and return to zero (or the
“0” is positive or zero voltage.
In case of sequentially “0” the voltage level
changes from positive to zero or zero to positive.
Manchester II Non Return to Zero (NRZ)
“1” is a change at half bit time from positive to
“0” is a change at half bit time from negative to
The most commonly used data bus in
Defines how avionics equipment and systems
communicate on the aircraft.
Unidirectional transition (simplex).
32 bits word transmitted over 2 wires (twisted
Bipolar RZ encoding. (“1”: +10V, “0”: -10V).
Messages are transmitted at 12.5 – 14.5 (low
speed mode) or 100 Kbps (high speed mode).
Up to 20 LRUs can be connected in a single
ARINC 429 bus.
Sequential words are separated by 4 bit times
High reliability, low weight and low cost.
However, limited data rates.
Is installed in:
Airbus A310 / A320
/ A330 / A340
Boeing 727 / 737 /
747 / 757 / 767.
Boeing 777 uses
The time needed for
a signal to rise from
10% to 90% of its
Parameter High Speed Low speed
Bit rate 100 Kbps 12.5 – 14.5 Kbps
1 bit time 10μsec ± 2.5% 1/(bit rate) μsec ± 2.5%
1/2 bit time 5μsec ± 5% 1/(bit rate/2) μsec ± 5%
Pulse rising time 1.5μsec ± 0.5% 10 ± 5 μsec
Pulse fall time 1.5μsec ± 0.5% 10 ± 5 μsec
2 kinds of word formats:
BNR (Binary): Data are encoded in binary numerical system.
example: 23 00010111
In BNR bit 29 indicates a positive / negative number, or North /
South, West / East, Above / Below. Bit 28 is the MSB.
BCD (Binary Coded Decimal): Each decimal digit is encoded
to the corresponding binary digit.
example: 23 0010 0011
BCD word encoding:
BNR word encoding:
P: Parity bit: Normally odd parity is used.
SSM (Sign / Status Matrix): Information about the data
characteristics (data content – e.g. test, validity of data, etc.)
Data: In BNR or BCD format.
Bits 11-13 can be used as an equipment identifier, if necessary, to
determine the equipment that transmitted the data. e.g. 00216 is the
FMC (Flight Management Computer).
SDI (Source / Destination Identifier): Source or Receiver
identification (e.g. does the data word targets a specific LRU or
every LRU connected on the bus?).
Label: The type of data and how to be translated. Usually
expressed in Octal. It is always sent first.
SSM for BCD data SSM for BNR data
BNR encoding example:
8 4 2 1odd
268 knots selected airspeed is transmitted through the ARINC
429 bus to every LRU connected in the bus.
BCD word format example:
Data transmitted: 25786 in decimal.
source of the data
BCD labels BNR labels
Decimal digits in BNR are
encoded as 1/2x
Used in Boeing 777.
Up to 120 LRUs can be connected. (46 are connected in
Clock speed: 100MHz.
Inductive coupling is used to connect the LRUs on
Data transferred to / from the bus using electromagnetic
Improved reliability since no break in the bus wires is
Data are transmitted to the bus in groups called
Each message consists of up to 31 word strings.
Each word string begins with a label word, followed by up
to 256 data words.
Each label word and data word is 20 bits.
Only one LRU is allowed to transmit data through the
bus each time.
One or more LRUs can receive data.
A time period common to all transmitters.
Every transmitter can make only one transition per terminal
A time period different to each transmitter. (Priority assignment).
Any transmitter is inactive until the terminal gap for that transmitter
A time period common to all transmitters, longer than the terminal
Will occur when all transmitters have had the chance to transmit.
Each transmitter can make only one transmission. Then, it must
wait until the synchronization gap has occurred, before it can
make a new transmission.
When an LRU is not willing to send data, the synchronization gap
2 modes of operation:
Periodic: LRUs transmit in order of power-up. (Normal
Aperiodic: LRUs transmit in priority order. Takes place
when a discrete event takes place.
e.g. Landing gear system down.
Other ARINC protocols
Used in Flight Data Recorder.
Harvard Bi-Phase encoding.
12 bit words of data.
Data are a snapshot of may avionics subsystems on the aircraft.
Each frame contains the same data at a different snapshot in time.
ARINC 717 is an alternative and extended protocol to ARINC 573.
Older specification of 429, now obsolete.
Used in airborne weather radar systems.
Simplex bus with 2 wires.
1 Mbps clock speed.
Data words 1600 bits long (64bit status word + 3x512 bits data).
A military half-dublex ARINC protocol.
2 twisted wires
Up to 30 terminals can be connected.
1MHz clock speed.
Word length: 20 bits. (16 bits are the data).
Manchester II bi-phase encoding.
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