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COMMUNICATIONS PROTOCOLS IN AERONAUTICS

by George Novacek

Start ý Data Bus Systems ý Timing is the Secret ý ARINC 429 ý CSDB and ASCB ý MIL-STD-1553B ý ARINC 629 and Beyond ý ARINC 429 Implementation ý Data Format ý Wrap Up ý Sources and PDF

DATA FORMAT

Now that you understand the hardware interface, letýs look at the data format. The following description explains the philosophy behind it. The protocol is thoroughly defined in the ARINC 429 specification, which, as I already warned, sports 520 pages. [3] The purpose of this article is to give you an overview of the protocol. Anyone who plans to design an ARINC 429 network needs to obtain the specification.

The ARINC 429 message consists of 32-bit words. The individual bit assignments are shown in Table 1. In the transmission, the word is sent LSB to MSB. To explain the bit assignment, Iýll start with the most significant bit, bit 32, which is a parity bit used for error detection. Bits 31 and 30 are Sign Status Matrix with four possible conditions: Failure Warning, No Computed Data, Functional Test, and Normal Operation.

Bits 29 through 11 are data bits, with bit 11 LSB and bit 29 MSB. Bit 29 also can be used for sign. The data bits must be padded with zeros. The data bits can carry different types of data: BNR, BCD, discrete data bits, maintenance, acknowledge, ISO alphabet, and even graphical.

Bits 10 and 9 are source/destination identifiers. Any member can be identified as a source (transmitter), sink (receiver), or source/sink (transmitter/receiver), even though it would be transmitting and receiving on separate lines.

Finally, the eight least significant bits, 1 through 8, hold a pointer to a look-up table where a corresponding six-character label can be found. This look-up table is a part of the ARINC 429 specification [3], and the labels listed there are standard (see Table 2). The three lowest characters are an octal code, followed by a three-character hex equipment ID. For example, in the label, the octal code 137 is dedicated to flap angle, label 270 to discrete data bits. The hex equipment ID 04A is for landing gear position unit, 04E is for fuel quantity indicator, and so on. In addition, the assignment of some bits, such as SSM, can change with the type of data being transmitted. Bit 11, for example, usually the data LSB, can be used to signify precision.

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