Avionics Data Buses Questions and Answers

Answer: A

Correct answer: Traffic policing at the switch

Each switch knows the bandwidth allocation gap and maximum frame size configured for every virtual link, and discards frames that exceed them. A babbling end system therefore has its excess traffic dropped at the first switch it reaches, and the rest of the network sees nothing unusual.

This is the robust partitioning that lets functions of different criticality share one network. The redundant second network protects against a failed link rather than a misbehaving source, and a CRC detects corruption rather than excess.

Answer: B

Correct answer: The bus is unidirectional with a single permitted transmitter

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

Two-way communication needs two buses, one in each direction. Speed, word length and parity are all irrelevant to the problem, which is topological: it is what the standard permits on the medium, not what the words can carry.

Answer: C

Correct answer: A bus monitor

A bus monitor receives every word on the bus and records it while transmitting nothing at all. Because it never puts anything on the wire, it cannot alter the timing or content of the traffic, so the system behaves during the test flight exactly as it does in service.

Each of the alternatives would transmit and so change what is being observed: a remote terminal would answer commands, a second controller would contend for the bus, and a network switch belongs to a quite different architecture.

Answer: D

Correct answer: unidirectional

Data on an ARINC 429 bus flows one way only, from the single permitted transmitter to as many as twenty receivers.

Everything characteristic of the standard follows from that. Nothing contends for the medium so there is no arbitration and no collision, the timing is trivially predictable, and a reply requires a second bus running the other way. Polling belongs to MIL-STD-1553, switching to ARINC 664, and bidirectional operation to neither - which is why a large aircraft accumulates so many 429 buses.

Answer: A

It occupies the first eight bits and is conventionally written in octal. Because it names the parameter rather than any equipment, a receiver simply listens for the labels it happens to need. No negotiation is required.

Answer: B

Correct answer: Manchester

Manchester biphase encoding places a transition at the middle of every bit, so the receiver sees an edge at a known point in each bit period and can lock its own clock to the incoming data.

No separate clock line is therefore needed. A second property matters as much: the signal has no DC component, which allows the transformer coupling the standard's stub connections depend on. The cost is bandwidth, the signalling rate being twice that of a code without guaranteed transitions.

Answer: C

Correct answer: virtual

A virtual link is a configured unidirectional path from one transmitting end system to a defined set of receivers, with a guaranteed maximum bandwidth. It lives in the switches' configuration tables rather than in any cable.

The idea reproduces ARINC 429's one-to-many broadcast on a switched network, while adding the rate limit that makes the timing analysable. Several virtual links share the same physical wiring, which is exactly what removes the wiring weight the older buses carried.

Answer: D

Correct answer: Determinism

Determinism means the worst-case time to deliver a message can be computed in advance and demonstrated, not merely observed to be good in testing.

Each standard achieves it differently: ARINC 429 by permitting one transmitter, MIL-STD-1553 by a controller holding the schedule, and AFDX by policing every source's rate so that queueing at the switches is bounded. Redundancy, integrity and partitioning are separate properties - a deterministic bus can still fail, which is why the flight-critical ones are duplicated.

Answer: A

Correct answer: Progressive relaxation of who may transmit and when

Read the access column across the four. ARINC 429 permits one transmitter, broadcasting continuously. MIL-STD-1553 permits many, but only when the controller commands. ARINC 629 removes the controller, terminals taking turns by distributed timing rules. ARINC 664 lets every end system transmit at will, subject to policing at the switches.

Each step gives sources more freedom and moves the guarantee of determinism elsewhere - from the topology, to a schedule, to a protocol, to enforcement in the network.

Answer: B

Correct answer: The sign status matrix

The sign status matrix reports whether the value is normal, a functional test result, no computed data or a failure warning. A word can arrive perfectly intact and still carry a value its own source has declared unusable.

Parity only tells the receiver the bits were not corrupted in transit; the label only tells it what the parameter is; the source destination identifier only says which receiver it is for. None of them says whether the value means anything, which is the SSM's job.