Arinc 429 – about transmissions

Arinc 429 – about transmissions

Arinc-429 is a dual-wired computer bus data transfer system. It is manufactured by Aeronautical Radio INC (ARINC), a dominant avionics data bus corporation. A mainstay in commercial avionics data bus transfer, arinc-429 is responsible for facilitating communication in commercial aircrafts’ equipment and data systems. Today, Arinc-429 is the commercial standard for aircraft avionics. It continues to be used in modern aircraft.

Its construction is simple: a single source transmitter that is connected to up to 20 receivers via a dual twisted wire. The data transfer is unidirectional, is simplex, but can be made to be bi- or multi directional if more channels are added. In these cases, each separate bus is an individual line replacement unit (LRU) and communicates with a different set of up to twenty receivers. The LRUs may be arranged in a star-shaped configuration, or in a branching straight-line arrangement called a bus drop. Each LRU can consist of multiple transmitters and receivers.

Transmission bus media wires are a 78 watt shielded twisted pair that need to be grounded at each end. The junctions from the wires to the bus also need to be grounded. The recommended power output should be 75 watts +/- 5 watts. This output needs to be equally split between the two wires.

There are two speeds for transmission: low speed and high speed. Low speed has a range of 12 to 14.5 kilohertz while high speed generally operates at 100 kHz. Transmission occurs in a bipolar format known as Return-to-Zero (RZ) modulation. RZ has 3 states (High, Null, and Low). The high state transmits at between +10.0 V and -1,0V and receives at between +6.5V and 13V. The null state transits at between 0V and 0.5V and receives at between +2.5V and -2.5V. The low state transmits at between -10.0V and +/- 1.0V and receives at between -6.5V and -13V.

Arinc-429 messages are typically one (data) word in length. Each word is a length of 32 bits. Bits in a data word are usually arranged from the last bit to the first bit. If the word is in a label field, the sequence is rearranged into an order that starts with the most significant bit. If the word is in a data field, the sequence is rearranged with the least significant bit first.

The 32nd bit acts as a parity bit that verifies clear transmission and reception of a message.

Bits 31-30 operate in the label field as the Sign/Status Matrix (SSM). They indicate and report the operating status of the source equipment.

Bits 11-29 are responsible for containing the word’s data information. They help indicate whether the data is binary (BNR), binary coded data (BCD) or discrete data.

Bits 9-10 are utilized by the Source/Destination Identifier (SDI). In this capacity, they identify from which source data is being transmitted or supposed to be sent to.

Bits 1-8 make up the label format or the Information Identifier. The label classifies and reports the word’s data type.