Data Buses, Computers and Power
Domain 10 covers the digital backbone: data buses, the computers that exchange data, and the electrical power that keeps them alive. Questions test bus characteristics, word structure, redundancy and the power architecture of a transport aircraft.
Data buses
- ARINC 429 - the classic avionics bus: unidirectional (one transmitter, up to about 20 receivers), 32-bit word, 12.5 or 100 kbit/s, label in octal, SDI, SSM and a parity bit. Simple and deterministic, but wiring-heavy.
- ARINC 629 - bidirectional and multi-transmitter, used on some Boeing types.
- MIL-STD-1553 - bidirectional, command/response, mostly military and some helicopters.
- AFDX - switched full-duplex Ethernet at 100 Mbit/s, with virtual links and redundancy management; used on modern Airbus types.
- CAN - cheap, robust, used for utility and system-level data.
Computers
Air data computers, IRUs, FMCs, flight control computers, display management computers and engine interface units. Each has BITE for fault detection and isolation; failures are annunciated and often handled by reversion or redundancy.
Electrical power
- 115 V AC, 400 Hz from engine-driven generators; 28 V DC from transformer-rectifier units and batteries.
- Buses are split into main, essential and emergency; non-essential loads are shed automatically on generator loss.
- A static inverter converts battery DC to AC for essential instruments; the RAT provides emergency hydraulic and electrical power; a GPU supplies ground power.
Worked example
An AFDX network carries a frame on both the A and B networks. The receiver accepts the first valid frame and discards the duplicate using a sequence number and integrity check, so a single cable or switch failure is transparent. A single ARINC 429 wire, by contrast, has no such redundancy; losing it loses the parameter.
Traps that cost marks
- Calling ARINC 429 bidirectional - it is not.
- Confusing AFDX redundancy management with simple duplication.
- Forgetting that essential and emergency buses survive generator loss and that shedding is automatic.
- Assuming a BITE message always means the component, rather than the wiring, is at fault.