Avionics Data Buses Questions and Answers

Answer: C

Correct answer: False, the bus is unidirectional with one transmitter

ARINC 429 defines one transmitter and up to twenty receivers, with data flowing one way. A receiver has no transmitter on that pair, so there is nothing to reply with, and two transmitters on one bus would corrupt each other.

Nor can they take turns: the standard provides no arbitration, no token and no scheduling mechanism to make alternation safe. Two-way communication needs two buses. Speed and parity are irrelevant - the limitation is what may be connected to the medium.

Answer: D

Correct answer: Any terminal may transmit whenever it has data

That describes a contention bus and is the opposite of MIL-STD-1553's design. Nothing transmits until the bus controller commands it, which is exactly what makes the traffic deterministic: no collisions, and a worst case that follows from the controller's schedule.

The other three are accurate. A bus controller directs all traffic, Manchester encoding allows clock recovery from the data, and dual redundant buses ensure a severed cable does not stop the system.

Answer: A

Correct answer: No, an even number of bit errors passes undetected

Parity makes the count of ones odd, so two errors - or four, or six - restore the original parity and the word passes the check.

The limitation matters because a burst of noise typically corrupts several adjacent bits at once, which is exactly the even case. The label being intact makes no difference, since parity is computed over the whole word. Parity is not optional in ARINC 429; it is simply weak, and the standard relies on rapid retransmission to make up for it.

Answer: B

Correct answer: Running the bus closer to its capacity

Higher loading degrades integrity rather than improving it: queueing delays grow, worst-case latency becomes harder to bound, and there is less room to absorb a burst of traffic without loss.

The other three all help. Checking the sign status matrix stops a receiver acting on a value the source has flagged as unusable; a CRC catches the even-numbered error patterns parity misses; and a redundant second bus keeps data flowing when a cable is severed. Each addresses a different threat - misuse, corruption and loss.

Answer: C

A monitor listens and records without transmitting, which is what lets it be added without disturbing the timing. A designated backup bus controller is the element that assumes control on failure. Only that unit can take over.

Answer: D

Correct answer: A central controller polling each end system

AFDX has no controller. Each end system transmits on its own virtual links whenever it has data, subject to the bandwidth allocation gap, and the switches police the result - determinism comes from bounding every source in advance rather than from a central authority granting turns.

The other three are integral to the design: virtual links as the logical paths, bandwidth allocation gaps as the rate limits, and dual redundant networks for availability. Central polling is MIL-STD-1553's approach.

Answer: A

It means the worst case delivery time can be bounded and demonstrated, nothing more. Reliability is addressed separately by redundancy, which is why deterministic buses are still duplicated. Timing and reliability are separate.

Answer: B

Correct answer: A lightly loaded bus transmits at a higher data rate

The data rate is fixed by the standard - 100 kbit/s on high-speed ARINC 429, 1 Mbit/s on MIL-STD-1553 - and does not vary with how much traffic is present. Bits go out at the same speed whether the bus is busy or idle.

The other three are the genuine reasons: existing timing guarantees stay undisturbed, later modifications can be accommodated without rewiring, and latency remains insensitive to small changes in traffic, which is what keeps the worst case demonstrable.

Answer: C

Noise, interference and hardware faults corrupt digital signals just as they do analogue ones. What digital transmission adds is the ability to detect the corruption rather than immunity from it. Detection is what changed.

Answer: D

Correct answer: The physical encoding used on the bus

Manchester encoding is fixed by the standard. It is a property of the physical layer and is unaffected by how many messages the schedule carries or how heavily loaded the bus is.

The other three all change. The controller's message schedule must be reworked to fit the new transfers in; the timing margins of existing transfers shrink as the schedule tightens; and whatever capacity remained for future growth is consumed. Those three are exactly why adding to a bus already at 75 per cent loading is difficult.