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) 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.

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).

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, …

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.

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..

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.

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.

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.

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.

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.

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.

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.

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.

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.

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

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:

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.

ARINC 429 SSM for BCD data SSM for BNR data

Lab

el lis

t (f

or

BN

R)

ARINC 429 BNR encoding example:

positive 25

6

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.

ARINC 429 BCD word format example:

Data transmitted: 25786 in decimal.

ARINC 429

Equipment identifier is optional.

Represents the source of the data word.

BCD labels BNR labels

Equipment Identifiers:

ARINC 429 TCAS Intruder Altitude Word

ARINC 429 Examples:

Decimal digits in BNR are encoded as 1/2x

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.

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.

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.

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.

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).

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.