Data Buses
ARINC 429
A 32-bit word on a twisted shielded pair: 8-bit label (quoted in octal), 2-bit source/destination identifier, 19 data bits, 2-bit sign/status matrix and 1 parity bit (odd). One transmitter only, up to 20 receivers, simplex, at 12.5 kbit/s (low speed) or 100 kbit/s (high speed). Because it is one-way, two-way conversation needs two buses.
Multi-transmitter buses
ARINC 629 (2 Mbit/s, time-division multiple access, current-mode coupling) allows many talkers on one bus. ARINC 664/AFDX is switched 100 Mbit/s Ethernet with virtual links and redundancy management. MIL-STD-1553 is 1 Mbit/s Manchester, command/response with one bus controller and up to 31 remote terminals, usually dual redundant. CAN, RS-422/485, RS-232 and I2C appear on smaller systems and maintenance equipment.
Traps
Two transmitters on one ARINC 429 bus is impossible, not merely bad practice. Watch the field widths, watch kbit/s against Mbit/s, and do not accept that AFDX is a simple linear bus. ARINC 429 labels are octal while the data may be BCD or binary (BNR): the label identifies the parameter and the SSM tells you whether the value is valid.
Rule of thumb
One talker means 429; many talkers means 629, AFDX or 1553. If the question mentions a bus controller, it is command/response (1553); if it mentions virtual links or Ethernet switches, it is AFDX.
Worked example
A technician wants a second radio altimeter transmitter connected to the same 429 receiver input on a display. The answer is that this is not permitted: use a second receiver port or another bus, because 429 allows only one transmitter per bus.