ARINC 429

ARINC 429 Data Bus Specification and Architecture

One of the most widely used bus standards used in commercial aircraft is the ARINC 429 data bus. The electrical and data properties and protocols are specified in the ARINC 429 specification.

ARINC 429 uses the Mark 33 Digital Information Transfer System, a unidirectional data bus standard (DITS). 32-bit packets of messages are transmitted at a bit rate of either 12.5 or 100 kilobits per second (referred to as low and high bitrate, respectively). Due to the unidirectional nature of the bus, separate ports, couplers, and cables are needed for an LRU to be able to transmit and receive data. Be aware that in a plane with advanced avionics, a lot of bus connections can be necessary.

A wide range of commercial transport aircraft, including the Airbus A310/A320 and A330/A340; the Boeing 737, 747, 757, and 767; and the McDonnell Douglas MD-11, have ARINC 429 installed on them. In order to lower the weight and size of the cabling and to enable faster data speeds than are achievable with ARINC 429, more recent aircraft, such as the Boeing 777 and Airbus A380, employ ARINC 629, which is a greatly upgraded bus specification. The ARINC 429 standard has shown to be quite stable, thus despite these shifts to faster, bidirectional bus standards, it is expected to stay in use for many years to come.

ARINC 429 Specification

For avionics in aircraft, ARINC 429 is a data transfer standard. It employs a self-synchronizing, self-clocking data bus protocol (Tx and Rx are on separate ports). Twisted pairs of wires used for the physical connection carry balanced differential signaling. Most communications consist of a single data word, which is 32 bits long. To other system elements that are watching the bus messages, messages are sent at 12.5 or 100 kbit/s. The NULL state or 32-bit data words are continuously transmitted by the transmitter (0 Volts). There can be no more than 20 receivers and one transmitter on a single wire pair. The protocol enables self-clocking at the receiver end, eliminating the requirement for clocking data transmission. An alternative to MIL-STD-1553 is ARINC 429.

Electrical characteristics

A single transmitter or source can be connected to a single receiver or sink using the two-wire differential ARINC 429 bus. There are two selectable speeds: 12.5 kbps and 100 kbps. 32-bit words are transmitted via the data bus using two signal wires. Sequential words are transmitted at least four times in bits apart (zero voltage). As a result, the system becomes selfclocking and no longer requires a separate clock signal. The ARINC 429 electrical characteristics are summarized below:

With either a positive or negative polarity, the nominal transmission voltage is 10V1V between wires (differential). Consequently, the range of each signal leg is +5 V to -5 V. Both conductors are at -5V if one is at +5V and vice versa. The 'A' (or '+' or 'HI') conductor is one wire, while the 'B' (or '-' or 'LO') wire is the other.

Bipolar return to zero (BPRZ) modulation is the type of modulation used. When measured between the conductors, the composite signal state may be at one of the following three levels:

HI, which should be between +7.25 and 11 volts (from point A to point B);
NULL, which should be between +0.5 and -0.5 volts (from point A to point B);
LO, which should fall between -7.25 V and -11 V (A to B).

The length of the line and the quantity of receivers connected to the bus affect the received voltage on a serial bus. A single bus with ARINC 429 shouldn't have more than 20 receivers linked to it. Each bus is unidirectional, therefore if a system needs to respond to or send messages, it must have its own transmit bus. Therefore, two distinct bus connections are required in order to provide bidirectional data flow.

Protocol

A twisted wire pair can only support one transmitter, hence ARINC 429 relies on a relatively straightforward point-to-point protocol. The transmitter either stays in the NULL state or continuously sends 32-bit data words. It should be noted that the ARINC specification supports up to 20 receivers, even though there may only be one receiver on a given bus wire.

Bit Timing and Slew Rate

The ARINC waveform's rise and fall times are referred to as the slew rate. More specifically, it describes the time required for the ARINC signal to increase from the 10% to 90% voltage amplitude points on the leading and following edges of a pulse. Both the high-speed and low-speed ARINC 429 systems are covered by the data.

ARINC 429 data word format

An ARINC message typically consists of one 32-bit data word. The type of data that is present in the rest of the word is specified by the 8-bit label field. The five basic fields of an ARINC data word—parity, SSM, data, SDI, and label—are all frequently present. ARINC data words are always 32 bits long. The bits are numbered by the ARINC standard from 1 (LSB) to 32. (MSB). There are numerous conceivable data formats.