ARINC 429, the "Mark 33 Digital Information Transfer System (DITS)," is the ARINC 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
Medium and signaling
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 (0 Volts). 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 an 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, a reversed ordering 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) for a 32-bit frame is conventionally diagrammed 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 ARINC 429 publications 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. Generally, when the ARINC 429 word format is illustrated with Bit 32 to the left, the numeric representations in the data field are 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.
Some equipment suppliers publish the bit transmission order as
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. Of note is 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 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" transceivers may be configured to reverse the Label field bit order "in hardware."
Word format
Each ARINC 429 word is a 32-bit sequence 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.