Avionics Data Buses Questions and Answers

Answer: A

Correct answer: Both Statement I and Statement II are correct

Both are true and the second gives the reasons. A bus filled close to capacity leaves nothing for the functions added over an aircraft's service life, and modifications are the norm rather than the exception.

It also degrades timing: as loading rises, queueing delays grow and the worst case becomes both larger and harder to bound, which is precisely what certification demands be demonstrated. Designers therefore work to loading targets well below saturation - often around half - treating the unused capacity as a design requirement rather than waste.

Answer: B

Correct answer: Statement I is incorrect but Statement II is correct

The second statement is right, and it refutes the first. Sharing a medium between functions of different criticality makes partitioning more necessary, not less: it must be shown that a fault in a low-criticality function cannot corrupt or delay a high-criticality one.

That is what traffic policing at the switches provides on an AFDX network - an end system transmitting faster than its allocation has its excess frames discarded at the first switch, so the fault is contained where it arises. Robust partitioning is the condition on which shared media are accepted at all.

Answer: C

Correct answer: 1, 2, 3, 4, 5

The 32-bit word is transmitted in that order. The label occupies bits 1 to 8 and is sent first, so a receiver can decide immediately whether the word concerns it.

The source destination identifier follows, then the data field, then the sign status matrix, and the parity bit is last at bit 32. Parity must come last because it is computed over everything before it. Note that the label's bits are themselves transmitted in reverse order within the field, a quirk of the standard's history.

Answer: D

Correct answer: 1, 2, 3, 4, 5

The sequence describes a controller-to-terminal transfer, and every step follows from the controller's authority. The controller issues a command word; that word names the terminal and the number of data words to follow, so the terminal knows what to expect.

The controller then sends the data words, and the terminal replies with a status word confirming receipt and reporting its own condition. Only then does the controller move to the next scheduled message. Nothing happens on the bus that the controller has not initiated, which is the source of the standard's determinism.

Answer: A

Correct answer: 1, 2, 3, 4

ARINC 429 came first, published in 1977 and still the most widely fitted civil bus. MIL-STD-1553, though dating from 1973 in military use, entered civil derivative applications afterwards.

ARINC 629 followed in the 1990s, notably on the Boeing 777, offering multiple transmitters without a controller. ARINC 664 part 7 came last, with the A380, taking commercial Ethernet and constraining it until its timing could be demonstrated. The sequence tracks a steady rise in bandwidth and in topological complexity.

Answer: B

Two independent networks carry every frame, and the receiver keeps the first copy to arrive. Redundancy management at the end system is what turns two ordinary networks into one reliable path. The first copy to arrive wins.

Answer: C

Correct answer: 1, 2, 3, 4

The order is one of increasing detection capability. A single parity bit catches any odd number of errors and no even number at all.

A checksum over the message catches more, since it depends on the values as well as their count, but is still fooled by compensating errors. A cyclic redundancy check is far stronger, detecting all burst errors shorter than its own length. Adding redundant transmission is stronger again, since a fault must corrupt two copies identically to pass. Each step costs bits and computation for the detection it buys.

Answer: D

Correct answer: Only three

The first three are correct: ARINC 429 has a single transmitter broadcasting to many receivers, MIL-STD-1553 has a bus controller directing remote terminals, and ARINC 664 is a switched network carrying virtual links.

The fourth contradicts the first. ARINC 429 permits exactly one transmitter per bus - that is the whole basis of its simplicity, since with no contention there is nothing to arbitrate. Where a list pairs one item with two incompatible descriptions, one of them must be the false entry.

Answer: A

Correct answer: Only three

The first three are right. ARINC 429 uses bipolar return to zero, with three voltage levels; MIL-STD-1553 uses Manchester encoding; and ARINC 664 uses the standard Ethernet physical layer, which is what lets it use commercial hardware.

The fourth contradicts the second by giving MIL-STD-1553 the wrong encoding. Manchester is essential to that standard because the guaranteed transition in every bit allows clock recovery and removes the DC component, which is what permits transformer coupling.

Answer: B

Correct answer: Only three

The first three describe their fields correctly: the label identifies the parameter, the sign status matrix reports validity and sign, and the parity bit detects an odd number of bit errors.

The fourth misstates the label as a receiver address. ARINC 429 has no addressing at all, because a single transmitter broadcasts to every receiver on the bus and each simply takes the labels it needs. Introducing an address would serve no purpose in that topology, which is why the field carries the parameter's identity instead.