ARINC 429 DITS (Digital Data Transmission System) is the prevailing technological standard for data transmission in aircraft electronics. It is used mostly for luxury commercial aircraft and transport aircraft.
ARINC 429 defines the physical and electrical interfaces of a two-wire data bus and a protocol data unit for supporting the local data network of on-board electronics found within an aircraft.
ARINC 429 is a data transmission standard used on aircraft in the field of aircraft electronics. Self-clocking and self-synchronisation of the data bus protocol are used. (Tcx and Rx are on different connections.) The physical connecting wires are twisted against each other and transmit a weighted, balanced signal. The data words have a 32 bit length and most messages consist of a single data word. The messages are transmitted with 12.5 or 100 kbit/s to other system elements that monitor the data bus messages. The transmitter continuously transmits either 32-bit data words or zeros. A connecting wire pair is limited to one transmitter and up to 20 receivers. The protocol permits self-clocking at the receiver end in order to avoid data clocking. ARINC 429 is an alternative to MIL-STD-1553.
Each ARINC 429 word is a single 32-bit value consisting of 5 fields:
- The bit 32 is the parity bit, and it is used to check whether the word has been damaged or made illegible during transmission. Each ARINC 429 channel normally uses odd parity, i.e. that the word must have an odd number of single bits. This bit is determined by 0 or 1 in order to make sure that the correct number of bits is determined by 1.
- Bits 30 to 31 in binary coded decimal (BCD) or bits 29 to 31 in the binary data word are the sign/status matrix or SSM. This frequently indicates whether the data in the word are valid or not.
- Normal operation (NO) - indicates that the data in this word can be considered as correct.
- Functional test (FT) - indicates that the data are provided by a test source.
- Failure warning (FW) - indicates an error pointing towards missing or illegible data.
- No computer data (NCD) - indicates that the data are missing or are faulty for some reason. For example, autopilot commands are indicated here as NCD, and not as FW when the autopilot is not switched on.
- The data sign (+/-) can also be shown, or any other information such as, for example, the orientation of the data (north/south; east/west).
- Bits 11 to 29 contain the data. The binary coded decimal (BCD) and binary number representation (BNR) bit fields are usual ARINC 429 data formats. The data formats also can be mixed.
- Bits 9 and 10 are source/destination identifiers (SDI) and indicate which receiver the data are meant for and often, which subsystem has transmitted the data.
- For identification of the data type, bits 1 to 8 contain a label based on the number 8. In practice, the bits are turned with the labels.
The label directives have been provided as a part of the ARINC 429 embodiment for different configurations. Each aircraft will contain a number of different systems, such as autopilot systems, air data acquisition systems, radar, altimeter, radios and GPS sensors. For each configuration type, a set of standard parameters has been specified that is usual for all models and makes. For example, every air data acquisition system will give the barometric altitude as label 203. This permits the exchange of certain parts to a certain extent, as most air data acquisition systems function in the same way or very similar. However, the number of labels is limited. For example, the label 203 could have a completely different meaning when sent from a GPS sensor. However, many frequently used aviation parameters use the same label independent of its source. In his products, each manufacturer also has slight deviations from the usual standard embodiment. Amongst others, examples for deviations include the provision of extra data which is not based on the standard, omission of the recommended data of the conventional embodiment and other changes of this kind.
Protection from Reception Disturbance
Systems in aeronautics must satisfy certain environment requirements, in most cases the RTCA DO-160 environment categories. ARINC 429 uses some physical, electronic, and protocol technologies to minimize electromagnetic interference with on-board radios and other equipment. For example, those involving other transmission cables.
The twisted cable pair is protected by an electrical resistance of 78 Ω. The ARINC 429 message transmission is specified for a difference of 10 V between data A and data B levels between the bipolar transmitters. As such, this specifies the raising and lowering of the voltage. For example, 5 V on data A and -5 V on data B would result in a valid signal.
The ARINC 429 uses a mutually complementary, differential, bipolar “return to zero” transmission wave (BPRZ) for data decoding in order to reduce the sensitivity of the cable itself with regard to parasitic currents. This means that the level has a voltage of +/- 10 V for the first half of the period, and 0 V for the other half.
Tools for Development
When developing and/or remedying faults of an ARINC 429 bus, the examination of the hardware can be very important for troubleshooting. Among other things, a logic analyzer with the ability to decode a code is a rather useful tool for collecting, analysing and decoding signals, so that the high-speed waveforms become visible.