Data Buses
Operation of data buses in aircraft systems, including ARINC 429 and its 32-bit word structure, ARINC 629, MIL-STD-1553B, ARINC 664/AFDX and CAN.
Data buses — summary notes
Free- A data bus is a shared serial highway that replaces point-to-point wiring between LRUs, cutting weight and connector count while allowing one parameter to be shared by many users.
- ARINC 429 is the classic civil bus: a single-source, multi-sink broadcast — one transmitter may feed up to 20 receivers, and a receiver can never talk back on that same bus. It uses a shielded twisted pair, bipolar return-to-zero (BPRZ) encoding with HI/NULL/LO states, and runs at 12.5 kbps (low speed) or 100 kbps (high speed).
- The ARINC 429 word is 32 bits: bits 1–8 the LABEL (read in OCTAL, identifying the parameter), 9–10 the SDI (source/destination identifier), 11–29 the DATA, 30–31 the SSM (sign/status matrix — valid, no computed data, functional test, failure warning) and bit 32 the PARITY bit, set for ODD parity.
- ARINC 629 is multi-source, multi-sink with no bus controller — terminals gain access by a timed protocol (CSMA/CA). It is the Boeing 777 bus, running at about 2 Mbps.
- MIL-STD-1553B is the military standard: a dual-redundant bus with a bus controller (BC) that commands remote terminals (RTs) in a command/response protocol, at 1 Mbps with Manchester bi-phase encoding. Nothing transmits unless the BC tells it to.
- ARINC 664 Part 7 / AFDX is switched full-duplex Ethernet with virtual links and guaranteed bandwidth, used on the A380, A350 and B787 — the backbone of integrated modular avionics.
- CAN bus (and ARINC 825) is a low-cost multi-master bus with message-priority arbitration, common in engine and utility systems.
- ⚠ Exam trap: ARINC 429 is single-source — if a question offers a bus where several LRUs transmit on the SAME wire pair, it is 629, 1553 or CAN, not 429. And the 429 label is octal, never decimal.
- ARINC 429 word
- bits 1–8 Label (octal) · 9–10 SDI · 11–29 Data · 30–31 SSM · 32 Parity (odd)
- ARINC 429 speeds
- 12.5 kbps (low) or 100 kbps (high)
- ARINC 429 fan-out
- 1 transmitter → max 20 receivers
- MIL-STD-1553B
- 1 Mbps · Manchester · BC + up to 31 RTs · dual redundant
Two air data computers must each supply altitude to three display units. How many ARINC 429 transmit buses are needed, and why?
Two — one per ADC. ARINC 429 permits only one transmitter per bus, so each ADC broadcasts on its own twisted pair; each display unit simply listens to both (a receiver may sink many buses). The two ADCs can never share one bus.
An ARINC 429 word arrives with the SSM set to "no computed data". How should the receiving LRU treat the data field?
As invalid. The SSM qualifies the data: the parameter must be rejected and the receiving system should flag or blank the associated indication rather than display a stale or meaningless value.
The ARINC 429 word
FreeData buses concept map
FreeData Buses
Data buses quiz
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