All about ARINC 429, a computer bus transfer system, the dominant factor in avionics data bus technology.

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ARINC 429 is a computer bus transfer system, classified as a two wire date differential. It is manufactures by Aeronautical Radio INC, or ARINC, as an alternative to MIL-STD-1553.

ARINC is the dominant factor in avionics data bus technology. Arinc 429 is one of ARINC's data transfer standards. As a dual wire differential, it can allow connectivity to one transmitter and can be a source for up to 20 receivers.

ARINC 429 is capapble of transmitting messages at 12.5 or 100 kilobits per second. Messaginges are typically one (data) word in length. The bit sequence is usually arranged form the last bit to the first bit. In label fields, the sequence is then rearranged into an order starting with the most significant bit first.

Bit Breakdown

Bit 32 operates as the parity bit and verifies that the message is clearly transmitted and received.
Bits 30 and 31 are the Sign/Status Matrix and are responsible for assigning encoding properties to the data.
Bits 11 through 29 are responsible for containing the data itself in several formats. These formats include Bit Field, Binary Coded Decimal (BCD), and Binary Number Representation (BNR). Bits 9 and 10 function as Source/Destination Identifiers (SDI) for the transmitting system. SDI identifies the source that is transmitting the data. It can also identify the receivers that the data is intended for. Bits 1-8 are octal label words that identify the type of data being transmitted.

Hardware Specifications and Configurations

ARINC 429 hardware configuration is rather simple. It is made up of one source transmitter that is connected via a dual twisted wire to between one and twenty sinks. The wire is generally shielded with up to 78 ohms of resistance and must be grounded at both ends. The sinks effectively function as receivers for the signals. Data transmission is unidirectional but can be reconfigured into bi-directional or multi-directional transmissions with additional channels or buses. There are two possible configurations: one is a star-shaped arrangement (star topology); the alternative is a simpler branching line arrangement (bus-drop topology). In either configuration, each separate device, or line replaceable unit (LRU) acts independently of the others. Each LRU contains its own set of transmitters and receivers and communicates on a different set of buses. The length of these arrangements varies, but can extend to well beyond 300 feet. Generally, lengths of 150 feet or less are the norm.

Transmission Specifications

ARINC 429 data transmission occurs in a bipolar, Return-to-Zero (RZ) modulation. It has three states (High, Null, and Low) and only two transmission speeds: 12 to 14.5 kHz for slow transmission speeds, and 100kHz +/- 100% for high speed transmissions. Each transmission state has a certain voltage range for transmission and reception. The High state transmits at between +10.0V and +/-1.0V and receives at between +6.5V and 13V. Null state transmits at between 0V and +/- 0.5V and receives at between +2.5V and -2.5V. Low state sends at between -10.0V and +/- 1.0V and receives at between -6.5V and -13V.