The ARINC 429 bus has been around now for over two decades and this article is intended to refresh us with its structure and definition. In the early 1980’s the ARINC Committee decided to take knowledge gained from ARINC specification 419 that had become a collection of several digital data protocols such as ARINC 561(LRN), 568(DME), 573(FDR) and 575(DADS) to name a few. The avionics systems evolving then where incorporating more computer components and requiring more processing functions. As a result of this growth a demand was seen to pass this volume of information to other systems. The “Mark 33 Digital Information Transfer System” (DITS) is a scheme where a source will transmit information out a designated port over a single twisted and shielded wire pair to multiple receivers. The committee realized a more comprehensive digital information transfer system could evolve from this philosophy that would allow multiple label groups to be sent. A single directional asynchronous bus that could be extremely flexible and expandable was required. Bi-directional data flow on a given data bus is not permitted and therefore separate transmit/receive ports would need to be assigned. This data format would also contain the clock within the bit stream and could be re-generated at the receiving equipment. There are five application groups for this 32 bit word and they are: BNR data, BCD data, discrete data, Maintenance data and acknowledgement. This new philosophy would require dedicated receiver and transmitter ports with different encoding formats. The first format use’s a two’s complement fractional binary notation (BNR) and the second format implements a Binary Coded Decimal (BCD) notation. The large majority of assigned labels implement BNR notation because it yields better resolution. This bus operates open loop in that it requires no feedback directly however some equipment manufacture’s through software will echo back some data from a receiver to confirm system integrity. The committee also recognized that data validity would be very important and so the bus structure would contain a parity bit.
The bus employs a bi-polar, return to zero (RZ) form of logic that is typically driven by +/-10v supplies. The bit rate employed within the arinc word can be in the range of 12.5-14.5kilobits/sec (Low Speed) or 100kilobits/sec (High Speed). The transfer rate or refresh rate that the source runs at is determined by its criticality within the affected systems. An example of different refresh rates can be seen for example in Label 204, Baro-Corrected Altitude, where the minimum transfer rate is 31msec vs. Label 012, which represents Ground Speed where its minimum transfer rate is 250msec. This specific detail is one that never comes to the forefront in any troubleshooting process however it does show the importance of the details embedded within the software handling the data. The transmitter port should be able to handle up to twenty receiver ports without any degradation in signal. The amount of data or label content on any particular transmitter port will vary depending on the equipment manufacturer and the systems that are interfaced.
The digital word is made up of 32 discrete bits with the least significant bit sent first. If in any word all locations are not used then they are filled in with binary zeros unless BNR/BCD numeric data is present and in this case valid data bits are used. Labels that contain numeric data by nature require more resolution and will typically use all available data bits. A case in point would be the previously mentioned Label 204 where the required data resolution is 17 bits (2℮17).