ARINC 429, "Digital Information Transfer System (DITS)," is the technical standard for the predominant avionics data bus used on most higher-end commercial and transport aircraft. It defines the physical and electrical interfaces of a two-wire data bus and a data protocol to support an aircraft's avionics local area network.
Technical description 
ARINC 429 is a data transfer standard for aircraft avionics. It uses a self-clocking, self-synchronizing data bus protocol (Tx and Rx are on separate ports). The physical connection wires are twisted pairs carrying balanced differential signaling. Data words are 32 bits in length and most messages consist of a single data word. Messages are transmitted at either 12.5 or 100 kbit/s to other system elements that are monitoring the bus messages. The transmitter constantly transmits either 32-bit data words or the NULL state. A single wire pair is limited to one transmitter and no more than 20 receivers. The protocol allows for self-clocking at the receiver end, thus eliminating the need to transmit clocking data. ARINC 429 is an alternative to MIL-STD-1553.
Word format 
Each ARINC 429 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. Every ARINC 429 channel typically uses "odd" parity - there must be an odd number of "1" bits in the word. This bit is set to 0 or 1 to ensure that the correct number of bits are set to 1 in the word.
- Bits 30 to 31 in the Binary Coded Decimal (BCD) or 29 to 31 in the binary data word is the Sign/Status Matrix, or SSM, and often indicates whether the data in the word is valid.
- Normal Operation (NO) - Indicates the data in this word is considered to be correct data.
- Functional Test (FT) - Indicates that the data is being provided by a test source.
- Failure Warning (FW) - Indicates a failure which causes the data to be suspect or missing.
- No Computed Data (NCD) - Indicates that the data is missing or inaccurate for some reason other than a 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). In this case it is considered to be in Normal Operation.
|SSM for BNR data|
|SSM for BCD data|
|Bit 31||Bit 30||Description|
|0||0||Plus, North, East, Right, To, Above|
|0||1||No Computed Data (NCD)|
|1||0||Functional Test (FT)|
|1||1||Minus, South, West, Left, From, Below|
- Bits 11 to 29 contain the data. Bit-field, Binary Coded Decimal (BCD), and Binary Number Representation (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. In practice, the bits representing the label are reversed.
The image below exemplifies many of the concepts explained in the adjacent sections.
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, radar 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 203. 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 203 may have some completely different meaning if sent by a GPS sensor, for example. 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.
Protection from interference 
Avionics systems are required to meet environmental requirements, usually stated as RTCA DO-160 environmental categories. ARINC 429 employs several physical, electrical, and protocol techniques to minimize electromagnetic interference with on-board radios and other equipment, for example via other transmission cables.
Its cabling is a shielded 78 Ω twisted-pair. ARINC signalling defines a 10Vp differential between the Data A and Data B levels within the bipolar transmission (i.e. 5V on Data A and -5V on Data B would constitute a valid driving signal), and the specification defines acceptable voltage rise and fall times.
ARINC 429's data encoding uses a complementary differential bipolar return-to-zero (BPRZ) transmission waveform, further reducing EMI emissions from the cable itself.
Development tools 
When developing and/or troubleshooting the ARINC 429 bus, examination of hardware signals can be very important to find problems. A logic analyzer with protocol decoding capabilities or protocol analyzers are useful tools which collect, analyze, decode, store signals so people can view the high-speed waveforms at their leisure.
See also 
- http://www.aviation-ia.com/AEEC (free to AEEC Members and Corporate Sponsors)