What is ARINC 429?

ARINC 429 is a data format for aircraft avionics. It provides the basic description of the functions and the supporting physical and electrical interfaces for the digital information system on an airplane. ARINC 429 is the predominant avionics data bus for most higher-end aircraft today. Heim system data recorders can record and play back this data format in its entirety. All original data is written to the media in the Heim DATaRec proprietary format and replayed in real time or at a faster speed if required.

Technical Description

ARINC 429 is a two-wire data bus that is application-specific for commercial and transport aircraft. The connection wires are twisted pairs. Words are 32 bits in length and most messages consist of a single data word. The specification defines the electrical and data characteristics and protocols. ARINC 429 uses a unidirectional data bus standard (Tx and Rx are on separate ports) known as the Mark 33 Digital Information Transfer System (DITS). Messages are transmitted at either 12.5 or 100 kbit/s to other system elements that are monitoring the bus messages. The transmitter is always transmitting either 32-bit data words or the NULL state. No more than 20 receivers can be connected to a single bus (wire pair) and no more than one transmitter.

ARINC 429 Word Format

Each ARINC word is a 32-bit value that contains five fields:

  • Bit 32 is the parity bit, and is used to verify that the word was not damaged or garbled during transmission.
  • Bits 30 to 31 is the Sign/Status Matrix, or SSM, and often indicates whether the data in the word is valid.
    • OP (Operational) - Indicates the data in this word is considered to be correct data.
    • TEST - Indicates that the data is being provided by a test source.
    • FAIL - Indicates a hardware failure which causes the data to be missing.
    • NCD (No Computed Data) - Indicates that the data is missing or inaccurate for some reason other than hardware failure. For example, autopilot commands will show as NCD when the autopilot is not turned on.
    • The SSM can also indicate the Sign (+/-) of the data or some information related to it like an orientation (North/South/East/West).
  • Bits 11 to 29 contain the data. Bit-field, Binary Coded Decimal (BCD), and two's complement binary encoding (BNR) are common ARINC 429 data formats. Data formats can also be mixed.
  • Bits 9 and 10 are Source/Destination Identifiers (SDI) and indicate for which receiver the data is intended or more frequently which subsystem transmitted the data.
  • Bits 1 to 8 contain a label (label words), expressed in octal, identifying the data type.

Labels

Label guidelines are provided as part of the ARINC 429 specification, for various equipment types. Each aircraft will contain a number of different systems, such as Flight Management Computers, Inertial Reference Systems, Air Data Computers, Radio Altimeters, Radios, and GPS Sensors. For each type of equipment, a set of standard parameters is defined, which is common across all manufacturers and models. For example, any Air Data Computer will provide the barometric altitude of the aircraft as label 204. This allows some degree of interchangeability of parts, as all Air Data Computers behave, for the most part, in the same way. There are only a limited number of labels, though, and so label 204 may have some completely different meaning if sent by a GPS sensor, for example. Many very commonly-needed aircraft parameters, however, use the same label regardless of source. Also, as with any specification, each manufacturer has slight differences from the formal specification, such as by providing extra data above and beyond the specification, leaving out some data recommended by the specification, or other various changes.

 

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