ARINC 429
ARINC 429 is a data bus standard for commercial aircraft that was developed in the 1970s and introduced in the 1980s . A distinction is made between a high-speed and a low-speed bus. With a high-speed bus, the maximum data transfer rate is 100 kbit / s, with low -speed 12.5 kbit / s.
There are other protocols based on the ARINC standard. ARINC 429 can also be found in a light version in business and sport aircraft . In military aviation and space travel, the MIL-STD-1553 bus is preferably used instead of the ARINC bus.
General
In principle, the serial interface ARINC-429 is comparable to the differential EIA-422 interface. Only three different levels have been defined instead of two (+10 V, 0 V and −10 V). How individual bits are transmitted can be seen in the representation of the resulting difference signal (ARINC-429 timing).
A characteristic of each bit is that the level has a voltage of ± 10 V for half the time and then 0 V in the other half ( RZ code ). Synchronization information is therefore available for every bit. An asynchronous interface e.g. B. RS-232 requires a start and stop bit for each data word for comparison.
The communication is a 1-to-n connection (one sender, n receiver). Due to the high voltage level, the same restrictions regarding cable length and transmission rate apply as for RS-232 and RS-422. Therefore, quite moderate transfer rates of either 12.5 ... 14.5 or 100 kilobits / second are defined. In the low-speed operating mode, tolerances of 10% apply and in the high-speed operating mode, tolerances of 5% apply to the transmission rate.
So-called slope control is used in order to avoid generating any interference signals through the transmission. This means that the rising and falling edges are as flat as possible in accordance with the transmission rate. Since there is no start bit, the initial recognition of a 32-bit data word takes place via a pause of at least 4 bits in which no signal is present (sync gap: minimum 4 bits). Within the 32 data bits there is only a gap of max. 2 bit periods allowed. And at the end of the data word, a gap of at least 4 bit periods is again defined.
Frame construction
Data transmission in the ARINC 429 always takes place in a fixed frame with a fixed length of 32 bits. The transmission of each frame begins with bit 1 and ends with bit 32, but the description in the standard always follows the direction from bit 32 to bit 1, i.e. in the opposite direction as the individual bits are physically transmitted. Each frame is divided into five areas that are permanently assigned to specific bit positions in the frame. These are:
- Bit 32: parity bit over the entire frame
- Bits 31 to 30 represent the Sign / Status Matrix (SSM) of the frame. Certain states are generally coded in this:
- Normal Operation (NO) - indicates that the data in the data field is assumed to be correct.
- Functional Test (FT) - indicates that the data in the data field is being transmitted for test purposes.
- Failure Warning (FW) - indicates that the data in the data field is assessed as not reliable due to a malfunction or is missing at all.
- No Computed Data (NCD) - indicates that the data in the data field is assessed as inaccurate or questionable for reasons other than a malfunction. For example, commands or messages from the autopilot are markedas NCD when the autopilot is not activated.
- Bits 29 to 11 are the actual data field in the frame. It can contain bit fields , numbers in BCD code or as two's complement or a combination thereof. The meaning and form are based on the label below .
- Bits 10 to 9 are the Source / Destination Identifiers (SDI) and express either the desired receiving module or the sending module of the frame.
- Bits 8 to 1 represent the label , which describes the data type in the data field. The different and possible labels are defined in a table in the standard.
literature
- Cary R. Spitzer, Uma Ferrell, Thomas Ferrell (Eds.): Digital Avionics Handbook . 3. Edition. CRC Press, 2015, ISBN 978-1-4398-6861-4 , chapter 34.
Web links
- ARINC website