1. Module 5: Digital Techniques and
Electronic Instrument Systems
5.4 Data Buses

2. What is a bus?
 Bus: A collection of wires
through which data is
transmitted from one part of a
computer (or one computer)
to another.
 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.

3. Address and Data Bus
 All busses consist of two
parts:
 Address bus: To which
device connected to the bus,
the data should go?
 Data bus: The actual data to
be transferred.
 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).

4. Serial vs. Parallel bus
 Parallel bus:
 Each bit of the data word
is transferred via a
specific wire.
 Requires a lot of wiring.
 Examples: conventional
PCI.
 Serial bus:
 Each bit of the data word
is transferred via the
same wire.
 Examples: PCIe, USB,
ARINC, I2C, …

5. Serial vs. Parallel bus
 Parallel buses
should have been
faster than serial.
 However:
 Parallel busses suffer
from clock skew. (i.e.
a bit can reach the
destination before or
after other bits: lack
of synchronization).
 Require more wiring.
 Cannot be
synchronized as fast
as the serial busses.
(lower data rate).
 Parallel buses are rare today.
Most busses architectures are
serial.

6. 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..

7. Communication Direction
 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.

8. Other bus characteristics
 Width: How many bits can be transmitted at the
same time.
 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.

9. ARINC
 ARINC: Aeronautical Radio
Incorporated.
 A company that develops and
operates aviation systems and
services.
 ARINC, ACARS (datalink between
aircraft and ground), LRU
standards.
 Develops also solutions for
defense, networks, security, …
 Founded in 1929.

10. Error detection
 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.

11. 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)
 Harvard Bi-Phase
 Manchester II Non Return to
Zero (NRZ)
 Self-clocking techniques:
 The clock is embedded in the
transmitted signal.
 The receiver LRU does not need
a clock to decode the data.

12. Bipolar Return to Zero (BPRZ)
 “1” is a positive voltage and return to Null at half bit
time.
 “0” is a negative voltage and return to Null at half bit
time.
 The return to Null is the way the receiver identifies
every single bit.
 Self-clocking: No clock is needed in the receiver.

13. Harvard Bi-phase
 “1” is positive voltage and return to zero (or the
opposite).
 “0” is positive or zero voltage.
 In case of sequentially “0” the voltage level
changes from positive to zero or zero to positive.

14. Manchester II Non Return to Zero (NRZ)
 “1” is a change at half bit time from positive to
negative voltage.
 “0” is a change at half bit time from negative to
positive voltage.

15. ARINC 429
 The most commonly used data bus in
commercial aircrafts.
 Defines how avionics equipment and systems
communicate on the aircraft.
 Characteristics:
 Unidirectional transition (simplex).
 32 bits word transmitted over 2 wires (twisted
pairs).
 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
Null voltage.
 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
ARINC 629.

16. ARINC 429
 Slew rate:
 The time needed for
a signal to rise from
10% to 90% of its
maximum voltage.
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

17. ARINC 429
 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:

18. ARINC 429
 Word Fields:
 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.

19. ARINC 429
 SSM for BCD data SSM for BNR data
Labellist(forBNR)

20. ARINC 429
 BNR encoding example:
positive
256
128
64
32
16
8 4 2 1odd
parity
Normal
operation 00.26841812561
Padded bits
Source /
Destination
(00 means
to
everybody)
1038:
Selected
airspeed
 268 knots selected airspeed is transmitted through the ARINC
429 bus to every LRU connected in the bus.

21. ARINC 429
 BCD word format example:
 Data transmitted: 25786 in decimal.

22. ARINC 429
 Equipment identifier
is optional.
 Represents the
source of the data
word.
 BCD labels  BNR labels
 Equipment Identifiers:

23. ARINC 429
 TCAS Intruder Altitude Word

24. ARINC 429
 Examples:
 Decimal digits in BNR are
encoded as 1/2x

25. ARINC 629
 Used in Boeing 777.
 Characteristics:
 Half Duplex.
 Up to 120 LRUs can be connected. (46 are connected in
Boeing 777).
 Clock speed: 100MHz.
 Inductive coupling is used to connect the LRUs on
the bus.
 Data transferred to / from the bus using electromagnetic
induction.
 Improved reliability since no break in the bus wires is
needed.

26. ARINC 629
 Data are transmitted to the bus in groups called
messages.
 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.

27. ARINC 629
 Terminal Interval.
 A time period common to all transmitters.
 Every transmitter can make only one transition per terminal
Interval.
 Terminal Gap.
 A time period different to each transmitter. (Priority assignment).
 Any transmitter is inactive until the terminal gap for that transmitter
has ended.
 Synchronization Gap.
 A time period common to all transmitters, longer than the terminal
interval.
 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
decreases.

28. ARINC 629
 2 modes of operation:
 Periodic: LRUs transmit in order of power-up. (Normal
operation).
 Aperiodic: LRUs transmit in priority order. Takes place
when a discrete event takes place.
 e.g. Landing gear system down.

29. Other ARINC protocols
 ARINC 573
 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.

 ARINC 575
 Older specification of 429, now obsolete.
 ARINC 708
 Used in airborne weather radar systems.
 Simplex bus with 2 wires.
 Manchester encoding.
 1 Mbps clock speed.
 Data words 1600 bits long (64bit status word + 3x512 bits data).

30. MIL-STD-1553B
 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.