Avionics Data Buses Questions and Answers

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 whole reason partitioning must be shown. Where a flight-critical function and a lower-criticality one share a medium, a fault in the latter could flood the network and delay the former, or corrupt data it depends on.

On AFDX the demonstration rests on traffic policing at the switches: a babbling end system has its excess frames discarded at the first switch, so the fault is contained where it arises. Without such a mechanism the shared medium could not be certified at all.

Answer: B

Correct answer: 1 and 2 only

The first two are correct: ARINC 429 has one transmitter broadcasting to up to twenty receivers, and its word is 32 bits.

The third is false. The word's only error detection is a single parity bit at bit 32 - there is no room in 32 bits for a CRC and none is defined. That is a real weakness, since parity misses every even number of errors, and the standard tolerates it because a discarded word is replaced by the next update within milliseconds.

Answer: C

Correct answer: 1 and 2 only

The first two are correct. The bus controller directs all traffic, and Manchester encoding puts a transition in every bit so the receiver can recover the clock from the data.

The third describes a contention bus and is the opposite of how MIL-STD-1553 works. No remote terminal transmits until commanded, which is precisely what makes the traffic deterministic: nothing collides, and the worst-case delivery time follows from the controller's schedule.

Answer: D

Correct answer: 1 and 2 only

The first two are correct. Virtual links reproduce a one-to-many broadcast on a switched network, and bandwidth allocation gaps cap the rate at which each may inject traffic.

The third belongs to a different architecture. AFDX has no central controller: each end system transmits when it has data, and determinism comes from every source being bounded in advance and the switches enforcing those bounds. Central polling is MIL-STD-1553's method, and it is what AFDX deliberately replaced.

Answer: A

Correct answer: 1 and 2 only

The first two are correct. Loading is held well below capacity to leave room for later modification and to keep latency bounded, and determinism means the worst-case delivery time can be computed and demonstrated.

The third confuses determinism with reliability. A deterministic bus can certainly fail - a severed cable stops it entirely, which is why MIL-STD-1553 and AFDX are both duplicated. Determinism says when a message arrives if it arrives, not that it always will.

Answer: B

Each design achieves determinism by a different mechanism while lifting a restriction of the one before it. Knowing which mechanism belongs to which bus is what makes their trade-offs comparable. Each solves the problem differently.

Answer: C

Correct answer: A-1, B-2, C-3, D-4

The four fields appear in the order they are transmitted and each answers a different question. The label says what parameter the word carries; the source destination identifier distinguishes among intended receivers.

The sign status matrix reports validity and sign, letting a source disclaim its own output; and the parity bit, last in the word, detects an odd number of bit errors. Note the division: two fields identify, one qualifies the value and one checks the transmission.

Answer: D

Correct answer: A-1, B-2, C-3, D-4

The four roles are distinguished by what each may put on the bus. The bus controller directs every transfer; a remote terminal transmits only when commanded; a bus monitor transmits nothing at all.

The backup bus controller is the one that can assume the directing role if the active controller fails - a capability the monitor deliberately lacks, since its passivity is what makes it safe to connect during a test flight without altering the system's behaviour.

Answer: A

Correct answer: A-1, B-2, C-3, D-4

The four are separate properties and confusing them is the trap. Determinism is a bounded and demonstrable worst-case delivery time; bus loading is the fraction of capacity in use.

A virtual link is a logical unidirectional path to defined receivers; partitioning is the demonstration that one function's fault cannot affect another. Note that determinism does not imply reliability, and partitioning is not redundancy - a network can be deterministic and still fail, or duplicated and still allow one function to starve another.

Answer: B

Correct answer: A cyclic redundancy check

The 32 bits are fully spoken for: eight for the label, two for the source destination identifier, nineteen for data, two for the sign status matrix and one for parity. There is no room for a CRC and none is defined.

Parity alone is therefore the word's entire error detection, which misses every even number of bit errors. ARINC 429 accepts that because a corrupt word is discarded and the parameter is retransmitted within milliseconds - repetition rate substituting for coding strength.