Jell AIARINC 429 has carried data between the line-replaceable units of commercial aircraft since the late 1970s. It grew out of ARINC 419 and the serial interface of the Digital Air Data System, and was first released as ARINC 429-1 in April 1978. Its formal name describes it well: Mark 33 Digital Information Transfer System, a simple, one-way path for digital data.
One talker, many listeners
An ARINC 429 bus is a single shielded twisted pair with a characteristic impedance of 78 Ω, driven by exactly one transmitter and read by one to twenty receivers. Data flows in one direction only; two units that need to talk both ways use two buses, and one unit may have several transmitters and receivers on different buses. The wiring is almost point to point and very reliable: there is no arbitration and no bus master that could fail.
Plus ten, minus ten, and back to zero
The transmitter drives its two wires, A and B, between three levels: HIGH at +10 V, NULL at 0 V and LOW at −10 V. Every bit is a pulse that lasts half the bit time: a 1 swings to +10 V, a 0 to −10 V, and both return to NULL for the second half. Because each bit ends at 0 V, the receivers recover the timing from the signal itself and no clock line is needed. Words are separated by at least four bit times of NULL, and between words the transmitter keeps sending NULL. A bus runs either at high speed, 100 kbit/s ±1 %, or at low speed, 12 to 14.5 kbit/s, never both.
The 32-bit word
Everything travels in 32-bit words:
- Label, bits 1 to 8: what the word carries, written as an octal number.
- SDI, bits 9 and 10: the optional source/destination identifier. It can tell which source sent the word or which receiver it is for, and some words use these bits for data instead.
- Data, bits 11 to 29.
- SSM, bits 30 and 31: the sign/status matrix. For binary data it reports the source’s status: 11 normal operation, 10 functional test, 01 no computed data, 00 failure warning.
- Parity, bit 32: odd parity over the whole word. ARINC 429 detects errors this way but does not correct them.
Binary (BNR) data is coded in two’s complement with the sign in bit 29; binary-coded decimal (BCD) data packs up to five digits. Most messages fit in a single word. The order of transmission is unusual: the label goes first, most significant bit first, and the rest of the word follows from bit 9 to bit 32, least significant bit first, a compensation for compatibility with legacy systems.
ARINC 429 on a test rig
On an aircraft systems test rig, the simulator both talks and listens. It plays the units that feed the equipment under test, sending each label at its update rate with the right SDI, SSM and data encoding, and it reads the equipment’s own output buses. SSM and parity are part of every word, which makes them natural places for fault injection: failure warnings, no computed data, wrong parity and missing labels, so you can check how the equipment reacts.
We integrate ARINC 429 into real-time systems for aircraft test rigs. If you are planning one, get in touch.
Sources
- AIM GmbH, ARINC 429 Tutorial, 2019, after ARINC Specification 429 Part 1