EASA Module 5 · 5.4 Data Buses

The ARINC 429 Word

Every ARINC 429 transmission is exactly 32 bits, always in the same field order. Build a word below and watch the label change in octal, the SSM qualify the data, and the parity bit adjust itself to keep the total number of 1s ODD.

Label (bits 1–8) 203₈

SDI — source/destination (bits 9–10) 00

Data field (bits 11–29) 0

Label (octal)

—

Data status

—

Parity bit 32

—

The label is read in octal, not decimal — three octal digits from the 8 label bits. The SSM qualifies the data: anything other than "normal operation" means the receiving system must reject or flag the value rather than display it. Parity is odd: bit 32 is set or cleared so the whole 32-bit word contains an odd number of 1s, which lets a receiver detect a single corrupted bit. Remember ARINC 429 is single-source — one transmitter, up to 20 receivers, and no reply on the same pair.