Avionics Data Buses Questions and Answers

Answer: A

A single twisted pair can carry hundreds of parameters where each once needed its own wire. On a transport aircraft the saving runs to hundreds of kilograms, which is why the change happened despite the added complexity.

Answer: B

Correct answer: Single transmitter, multiple receiver unidirectional bus

ARINC 429 allows exactly one transmitter on a bus, broadcasting to up to twenty receivers, and data flows one way only. There is no arbitration because there is nothing to arbitrate.

That simplicity is the standard's great strength and its great cost. Nothing can collide and the timing is trivially predictable, but two-way communication needs two buses, and a large aircraft accumulates a great many of them. MIL-STD-1553 uses a controller with remote terminals, and ARINC 664 is the switched network with virtual links.

Answer: C

The word divides into a label, a source destination identifier, the data field, a sign status matrix and a parity bit. That fixed structure is what lets any receiver decode a word without negotiating with the transmitter.

Answer: D

Correct answer: The parameter the word carries

The eight-bit label says what the word contains - altitude, airspeed, heading - rather than who sent it or who should read it.

The design follows from the topology. With one transmitter broadcasting to every receiver on the bus, an address would be pointless: each receiver simply examines the label and takes the words it needs. Labels are assigned by the standard, conventionally written in octal, so equipment from different manufacturers interprets the same label the same way.

Answer: A

Correct answer: The validity and sign of the data

The two-bit sign status matrix qualifies the data field, reporting whether the value is normal operation, functional test, no computed data or failure warning, and for some formats carrying the sign.

Its importance is that it lets a source disclaim its own output: an air data computer that has lost a probe still transmits, but flags the word as no computed data. A receiver that reads the data field without checking the SSM will act on a value the transmitter has already declared unreliable.

Answer: B

Correct answer: Odd parity in the final bit of each word

Bit 32, the last transmitted, is set so that the total number of ones in the word is odd. A receiver counting an even number knows the word is corrupt and discards it.

The scheme is deliberately minimal and correspondingly weak: it detects any odd number of bit errors and misses every even number, and it cannot correct anything. The standard accepts that because a corrupt word is simply dropped and the next update follows within milliseconds - a repetition rate that substitutes for stronger coding.

Answer: C

Low speed suits slowly changing parameters and high speed the rapidly changing ones. A receiver must be configured for the rate in use, since the bus carries no indication of which is running. The receiver must be configured. Configuration must match the bus.

Answer: D

A null state between every bit makes the clock recoverable and removes any direct current component from the line. The three states are a positive pulse, a negative pulse and the null between them. The null carries the clock.

Answer: A

Nothing transmits until the controller commands it, which makes the traffic entirely deterministic. The cost is that the controller becomes critical, so a backup controller is normally provided. Central control has a price.

Answer: B

Correct answer: Listens to all traffic without transmitting

A bus monitor is a passive device. It receives every word on the bus and records or analyses it, and transmits nothing at all.

Passivity is the point: because it never puts anything on the wire it cannot alter the timing or the content of the traffic, so the system behaves during a test exactly as it does in service. That is what makes it the right instrument for flight test recording and for maintenance analysis, where an active terminal would change what it was trying to observe.