Avionics Data Buses Questions and Answers

Answer: C

Correct answer: Manchester encoding

MIL-STD-1553 uses Manchester biphase encoding, in which each bit is transmitted as a transition at mid-bit - low to high for one value, high to low for the other.

Two properties follow and both matter. A transition in every bit lets the receiver recover the clock from the data itself, so no separate clock line is needed. And the signal has no DC component, so it can be transformer-coupled, which gives the galvanic isolation the standard's stub couplers rely on.

Answer: D

Correct answer: Providing dual redundant bus pairs

The standard specifies two independent buses, A and B, each with its own wiring and transformers. Terminals connect to both, and the bus controller uses one at a time, switching to the other if a transfer fails.

The redundancy is therefore in the medium, which is where the vulnerability lies - a severed or shorted cable takes out a bus, not a terminal. Transmitting every message twice would halve the throughput without protecting against a broken wire, and duplicating terminals addresses a different failure altogether.

Answer: A

Sixteen data bits are wrapped in a three bit time sync pattern and a parity bit. The sync pattern is deliberately not a valid Manchester code, which is what makes the start of a word unmistakable. The receiver cannot mistake it.

Answer: B

Terminals wait out timing intervals that guarantee each gets access without collision. Removing both the single transmitter restriction and the central controller was the point of the design. Both restrictions went at once.

Answer: C

Correct answer: Avionics full duplex switched Ethernet

ARINC 664 part 7 defines AFDX, a deterministic profile of switched Ethernet developed for the A380 and now widespread on large transports.

It takes commercial Ethernet hardware and constrains it until the timing can be guaranteed: traffic travels on virtual links with a policed maximum rate, switches enforce that policing, and the network is duplicated for redundancy. The result keeps Ethernet's bandwidth and its component supply while acquiring the bounded latency that certification requires.

Answer: D

Correct answer: A logical path to defined receivers

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

The concept is what makes a switched network deterministic: because every link's traffic is bounded in advance and the switches police it, the worst-case delay across the network can be computed and demonstrated. Several virtual links share the same physical wiring, which is precisely the point.

Answer: A

Enforcing a minimum spacing caps the rate at which any link can inject traffic, so no single source can flood the network. Bounding each contributor is what makes the worst case latency calculable. Every source is bounded.

Answer: B

A flight control law needs to know its data will arrive within a stated time, not merely that it will arrive eventually. Ordinary office networking makes no such guarantee, which is exactly what the aviation adaptations add.

Answer: C

Correct answer: The proportion of available capacity actually in use

Bus loading is the ratio of traffic carried to capacity available, usually expressed as a percentage.

It matters for two reasons. A heavily loaded bus has little margin for later modification, and integrators expect to add functions over an aircraft's life. And as loading rises, queueing delays grow and worst-case latency becomes harder to bound - which is what certification requires. Designers therefore aim well below saturation, commonly around 50 per cent, rather than filling the available capacity.

Answer: D

Functions of different criticality may share one physical network, so the design must show that a failure cannot propagate between them. Guaranteed bandwidth and traffic policing are the usual means of demonstrating it. Both are demonstrable in test.