ARINC 429 Data Bus

Direct Answer

The ARINC 429 standard is the predominant two-wire differential data bus used across commercial and transport avionics, transmitting 32-bit digital words over a single transmitter / multi-receiver shielded twisted pair using Bipolar Return-to-Zero (BPRZ) modulation.

Specs

Specification	Aeronautical Radio, Inc. (ARINC) 429 Mark 33 Digital Information Transfer System
Physical Topology	Single transmitter, up to 20 receivers (simplex broadcast, point-to-point)
Wiring Medium	78 Ω shielded twisted pair cable
Low-Speed Baud Rate	12.5 kbps (bit duration 80 μs)
High-Speed Baud Rate	100.0 kbps (bit duration 10 μs)
Modulation Scheme	Bipolar Return-to-Zero (BPRZ: +10V High, -10V Low, 0V Null)

ARINC 429 32-Bit Word Protocol Architecture

ARINC 429 operates on a single transmitter broadcasting to up to 20 receivers across a 78, Ω shielded twisted pair using Bipolar Return-to-Zero (BPRZ) differential signaling:

Bit 1 ----------------------- Bit 8 | 9-10 | 11 ------------------------ 29 | 30-31 | Bit 32
[ Label (8 Bits) ] [SDI] [ Payload Data (19 Bits) ] [ SSM ] [Parity]

Bits 1–8: Label: Identifies the parameter being transmitted (e.g. 203 = Altitude, 206 = Airspeed, 310 = Latitude). Coded in reverse octal with MSB first.
Bits 9–10: Source/Destination Identifier (SDI): Specifies which system is transmitting (e.g. 00 = all systems, 01 = Left/Capt system, 10 = Right/FO system, 11 = Center system).
Bits 11–29: Data Payload (19 Bits): Transmitted in BNR (Binary Two's Complement) for continuous variables or BCD (Binary Coded Decimal) for discrete parameters.
Bits 30–31: Sign/Status Matrix (SSM): Indicates operational status (00 = Normal/North/Plus, 01 = No Computed Data / East, 10 = Functional Test, 11 = Failure Warning / South / Minus).
Bit 32: Parity Bit: Odd parity bit ensuring total count of logical 1s in the 32-bit word is odd.

Essential ARINC 429 Parameter Label Directory

Label (Octal)	Parameter Name	Encoding Format	Resolution / Units	Transmit Rate
`203`	Pressure Altitude	BNR	1.0 ft (-1,000 to +131,072 ft)	64 Hz
`204`	Baro Corrected Altitude	BNR	1.0 ft (-1,000 to +131,072 ft)	64 Hz
`205`	Mach Number	BNR	0.000061 Mach (0.0 to 4.0 M)	32 Hz
`206`	Computed Airspeed (CAS)	BNR	0.0625 knots (0 to 1,024 kt)	32 Hz
`210`	True Airspeed (TAS)	BNR	0.0625 knots (0 to 2,048 kt)	32 Hz
`212`	Vertical Speed	BNR	1.5625 ft/min (-32,768 to +32,768 fpm)	64 Hz
`310`	Present Position: Latitude	BNR	0.000172° (-90° to +90°)	10 Hz
`311`	Present Position: Longitude	BNR	0.000172° (-180° to +180°)	10 Hz
`312`	Ground Speed	BNR	0.125 knots (0 to 4,096 kt)	10 Hz
`314`	True Heading	BNR	0.0055° (0° to 360°)	20 Hz
`320`	Magnetic Heading	BNR	0.0055° (0° to 360°)	20 Hz