Avionics Data Buses Questions and Answers

Answer: C

Correct answer: Slowly varying parameters do not need the faster rate

Fuel quantity, configuration discretes and similar parameters change over seconds, so an update every few milliseconds serves no purpose. Sending them at the low rate uses a fraction of the bandwidth for the same usefulness.

The choice is about bandwidth economy rather than capability: receivers decode whichever rate their bus runs at, and both rates use the same wiring and the same word format. The table's ratio column makes the point - the high rate costs eight times the capacity per parameter.

Answer: D

Correct answer: It misses an even number of errors

Parity works by making the total count of ones odd, so two errors, or four, restore the original parity and the word passes as good.

That is a serious limitation where a burst of noise typically corrupts several adjacent bits at once. ARINC 429 accepts it because a discarded word is replaced within milliseconds by the next update, but a bus carrying data that is not repeated at that rate needs a CRC. Computation speed and bit cost both favour parity, so neither can be the reason it was abandoned.

Answer: A

Correct answer: A larger transfer must be split across several messages

The command word's word count field is five bits, encoding 1 to 31 directly and using zero for 32, so no single message can carry more.

A larger data set is therefore sent as a sequence of messages, each with its own command and status word. That adds overhead but does not limit what the bus can carry - the second option overstates the consequence. Nothing about the limit requires a duplicate controller or a longer word; it is a field width, and the protocol works around it.

Answer: B

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the second is the reason for the first. An ARINC 429 bus permits exactly one transmitter and carries data one way, so a receiver has no means of putting anything on the same wires.

Two-way communication therefore needs two buses. That is the standard's principal cost: a large aircraft accumulates hundreds of them, with the wiring weight and installation labour that implies, and it is exactly what ARINC 629 and later the switched networks were designed to escape.

Answer: C

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the second explains why the check is necessary. Parity confirms only that the bits survived transmission; it says nothing about whether the value they encode means anything.

The sign status matrix is how a source disclaims its own output - an air data computer that has lost a probe keeps transmitting but flags the word as no computed data. A receiver reading the data field without the SSM will act on a value the transmitter has already declared unreliable, which is a real and recurring integration fault.

Answer: D

Both are true and the second is the mechanism. Response gaps are specified tightly, so the controller can schedule the entire bus in advance rather than reacting to what terminals happen to do. The schedule is fixed in advance.

Answer: A

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the second is the mechanism. A transition at the middle of every bit means two signal elements per bit period, so the highest fundamental frequency equals the bit rate rather than half of it.

A 1 Mbit/s Manchester link therefore needs about 1 MHz of signalling bandwidth where a non-return-to-zero code would need 500 kHz. The bandwidth buys clock recovery from the data and a DC-free signal, which is what permits the transformer coupling MIL-STD-1553 uses.

Answer: B

Both hold and the second is the mechanism. Once every contributor is bounded, a worst case latency can be computed and demonstrated, which is precisely what a flight control function requires. Eventually is not good enough.

Answer: C

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the second gives the principal reason. Aircraft are modified throughout their service lives, and a schedule filled close to capacity leaves nothing for the functions added later - a bus that cannot take another message forces new wiring instead.

A second reason reinforces it: as loading rises, queueing delays grow and worst-case latency becomes both larger and harder to bound, which is what certification requires be demonstrated. The unused capacity is a design requirement, not slack.

Answer: D

Correct answer: A is false but R is true

The reason is correct as far as it goes: parity does detect any odd number of bit errors. But it refutes the assertion rather than supporting it, because the complement is that it detects no even number at all.

Two corrupted bits pass undetected, and noise bursts commonly corrupt several adjacent bits at once. Parity is adequate on ARINC 429 only because a discarded word is replaced within milliseconds; data not repeated at that rate needs a CRC, which is why later standards use one.