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.
Medium, Signaling, and Topology
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.
Bit numbering, Transmission Order, and Bit Significance
The ARINC 429 unit of transmission is a fixed-length 32-bit frame, which the standard refers to as a 'word'. The bits within a ARINC 429 word are serially identified from Bit Number 1 to Bit Number 32 or simply Bit 1 to Bit 32. The fields and data structures of the ARINC 429 word are defined in terms of this numbering.
While it is common to illustrate serial protocol frames progressing in time from right to left, the reversed order is commonly practiced within the ARINC standard. Even though ARINC 429 word transmission begins with Bit 1 and ends with Bit 32, it is common to diagram and describe  ARINC 429 words in the order from Bit 32 to Bit 1. In simplest terms, while the transmission order of bits (from the first transmitted bit to the last transmitted bit) is conventionally diagramed as
- First bit > 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, ... 29, 30, 31, 32 < Last bit,
this sequence is often diagrammed in the opposite direction as
- Last bit > 32, 31, 30, 29, ... 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1 < First bit.
When the ARINC 429 word format is illustrated with Bit 32 to the left, the numeric representations in the data field generally read with the Most significant bit on the left. However, in this particular bit order presentation, the Label field reads with its most significant bit on the right. Like CAN Protocol Identifier Fields, ARINC 429 Label fields are transmitted most significant bit first. However, like UART Protocol, Binary-coded decimal numbers and binary numbers in the ARINC 429 data fields are generally transmitted least significant bit first.
- First bit > 8, 7, 6, 5, 4, 3, 2, 1, 9, 10, 11, 12, 13 … 32 < Last bit.
The suppliers that use this representation have in effect renumbered the bits in the Label field, converting the standard’s MSB 1 bit numbering for that field to LSB 1 bit numbering. This renumbering highlights the relative reversal of "bit endianness" between the Label representation and numeric data representations as defined within the ARINC 429 standard. Observe how the 87654321 bit numbering is similar to the 76543210 bit numbering common in digital equipment; but reversed from the 12345678 bit numbering defined for the ARINC 429 Label field.
This notional reversal also reflects historical implementation details. ARINC 429 transceivers have been implemented with 32-bit shift registers. Parallel access to that the shift register is often octet-oriented. As such, the bit order of the octet access is the bit order of the accessing device, which is usually LSB 0; and serial transmission is arranged such that the least significant bit of each octet is transmitted first. So, in common practice, the accessing device wrote or read a "reversed label" (for example, to transmit a Label 2138 [or 8B16] the bit-reversed value D116 is written to the Label octet). Newer or "enhanced" transcievers may be configured to reverse the Label field bit order "in hardware".
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 are the Sign/Status Matrix (SSM) in all cases using that function and may be encoded to indicate:
- 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.
- In case of Binary Coded Decimal (BCD) representation, the SSM may also indicate the Sign (+/-) of the data or some information related to sign, like an orientation (North/South; East/West). In this case it is considered to be in Normal Operation.
- In the case of the standard's two's-complement representation of signed binary numbers (BNR), Bit 29 represents the number's sign—sign indication is delegated to Bit 29 in this case.
- In cases of discrete data words, the SSM has a different, signless encoding.
|SSM||Data Dependent SSM Encodings:|
|Bit 31||Bit 30||Sign/Status Matrix for BCD Data||Status Matrix for BNR Data||Status Matrix for Discrete Data|
|0||0||Plus, North, East, Right, To, Above||Failure Warning (FW)||Verified Data, Normal Operation|
|0||1||No Computed Data (NCD)||No Computed Data (NCD)||No Computed Data (NCD)|
|1||0||Functional Test (FT)||Functional Test (FT)||Functional Test (FT)|
|1||1||Minus, South, West, Left, From, Below||Normal Operation (NO)||Failure Warning (FW)|
|Bit 29||Sign Matrix for BNR Data|
|0||Plus, North, East, Right, To, Above|
|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 may also be mixed.
- Bits 9 and 10 are Source/Destination Identifiers (SDI) and may indicate the intended receiver or, more frequently, indicate the transmitting subsystem.
- 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.[With Respect to What?]
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.
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.
- Steve Woodward (July 11, 2002). Bill Travis, ed. Circuit transmits ARINC 429 data (PDF). EDN Magazine.
- "ARINC 429 Bus Interface" (PDF). Actel. Retrieved 2009-06-24.
- ARINC Specification 429, Part 1-17. Annapolis, Maryland: Aeronautical Radio, Inc. 2004-05-17. pp. 2–5.
- ARINC Specification 429, Part 1-17. Annapolis, Maryland: Aeronautical Radio, Inc. 2004-05-17. pp. 78–116.
- ARINC 429 Protocol Tutorial. Avionics Interface Technologies. pp. 13–21.
- Novacek, George (May 2001). "Communications Protocols in Aeronautics". Circuit Cellar (Online). p. 5.
- CAN Specification 2.0, Part B. CAN in Automation. p. 9.
- ARINC429 Specification Tutorial. Freiburg, Germany: AIM GmbH. p. 15.
- ARINC Protocol Tutorial. Santa Barbara, CA: Condor Engineering, Inc. 2000. p. 9.
- HI-8783, HI-8784, HI-8785 ARINC 429 & 561 Interface Device. HOLT Integrated Circuits, Inc. 2009. pp. Figure 1 : Block Diagram.
- ARINC 429 Programming Manual. Ballard Technology. pp. A–2.
- HI-3584 Enhanced ARINC 429 3.3V Serial Transmitter and Dual Receiver (Rev G.). HOLT Integrated Circuits, Inc. 2013. p. 4.
- ARINC Specification 429, Part 1-17. Annapolis, Maryland: Aeronautical Radio, Inc. 2004-05-17. pp. 3–5.
- http://www.aviation-ia.com/AEEC (free to AEEC Members and Corporate Sponsors)