ARINC 429’s data transmission is unidirectional on a single channel or bus. This standard is known as Mark 33 Digital Information Transfer System (DITS).
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ARINC, or Aeronautical Radio Incorporated, is a private corporation that specializes in manufacturing aircraft and avionics equipment. ARINC also publishes standards and specifications for the Airlines Electronic Engineering Committee (AEEC).
The specifications for ARINC 429 deal with electrical characteristics, bus communication, and data protocol. This defines the communications between avionics equipment. ARINC 429’s data transmission is unidirectional on a single channel or bus. This standard is known as Mark 33 Digital Information Transfer System (DITS). This is a simplex, twisted shielded pair data bus which transmits data messages at a bit rate of 12.5 or 100 kilobits per second.
A basic ARINC 429 bus consists of one source transmitter and between 1 and 20 connected receivers or sinks, all on the same twisted pair. The data bus requires two signal wires to transmit 32 bit words. Each bus is known as a line replaceable unit or LRU and may contain multiple transmitters and receivers. The LRU’s can be arranges in one of 3 designs: the star topology, where the receiving LRU’s are connected in a ring to a central transmitter; the bus-drop topology, where the LRU’s are arranged in a series with the transmitter as the head; and multiple bus designs with 2 or more LRU’s connected to several singular receivers. Communications between transmitters and receivers is known as “handshaking” and requires 2 or more twisted pairs to pass data back and forth across two channels. The transmissions send words made of 32 bits. This is further broken down into 24 bits for actual information and an 8 bit label that provides a data description.
Transmissions are sent as 10w speed (12-14.5 kHz) or high speed (100 kHz). It is sent in a bipolar, Return-to-Zero format with a tri-state modulation (High, Null, Low states). The high state transmits between +10.0 V and +/- 1.0 V and receives a range from +6.5 V to 13V. The null state transmits between 0V and +/- 0.5V and receives from +2.5 to -2.5V. The low state transmits -10.0V to +/- 1.0V and receives -6.5 to -13V. The shielded twisted pair cable has a resistance of 78 ohms and has to be grounded at both ends and at each junction throughout the bus. Generally, systems are designed for length between 150 and 300 feet, but can be extended beyond this in some instances.
The ARINC 429 data words are composed of five fields. The parity field contains 1 bit and is described as the Most Significant Bit (MSB). Like most computers, ARINC uses odd parity to check the data reception for any errors.
The Sign/Status Matrix (SSM) field contains bits 30 and 31. They are used for interpreting the direction of ARINC words.
The data information field is contained in bits 11-29. The configuration of these bits is what determines whether the data is Binary (BNR), Binary Coded Decimal (BCD), or discrete data.