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MX Foundation 4
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MX Foundation 4
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ARINC 429, "Mark 33 Digital Information Transfer System (DITS)," is a technical specification for an avionics data bus used on most high-end commercial and transport aircrafts.
At the physical layer, it features communication of data words over unidirectional, two-wire, bipolar electrical interface. Bit-encoding and decoding is controlled by a self clocking RZ signal and self-synchronizing data bus protocol. The transmit and receive channels are on separate ports.
In transmit, a single wire pair has one transmitter and supports up to 20 receivers. ARINC 429 data words are 32 bits (sometimes 25 bits), and most messages consist of a single data word. Messages are transmitted at either low speed (12.5 kbit/s) or high speed (100 kbit/s) to receiver elements. The transmitter constantly transmits either 32-bit data words or the NULL state.
The basic transmission unit for the ARINC 429 protocol is a 32-bit value called a word.
The word is defined as follows:
| Word | Description | Usage |
|---|---|---|
| Bit 32 | Parity Bit | Parity Bit (for Integrity Check) |
| Bits 31-30 | SSM | Signed Status Matrix (in BCD or BNR) |
| Bits 29-11 | Data | Data to be transmitted (bit 29 used if BCD) |
| Bits 10-9 | SDI | Source Destination Identifier |
| Bits 8-1 | Label | Identify the data type: value in octal |
At the physical layer the data words are transmitted on the data bus the label first (MSB) and the others 24 bits are sent in LSB reversed order.
This means that the word bits are transmitted in sequence as follows
[8 7 6 5 4 3 2 1 9 10 11 12 13 ... 32]
Because the label is transmitted first, the receiving LRU then knows what type of data to deal with in the incoming bits that follows [9 10 11 ... 32].
Typically the data types used by ARINC 429 are:
Binary Number Representation (BNR): In fractional binary two's complement.
BNR encoding stores data as a binary number.
Binary Code Decimal (BCD): Numerical Subset of ISO Alphabet No 5
BCD encodes each decimal value in 4-bit digit.
and other less used data types:
Depending on the data type defined by the label the Sign/Status bit mapping is as follows:
Sign/Status Matrix (BCD format)
| Bit 31 | Bit 30 | Meaning (BCD Numeric) |
|---|---|---|
| 0 | 0 | Plus, North, East, Right, To, Above |
| 0 | 1 | No Computed Data (NCD) |
| 1 | 0 | Functional Test (FT) |
| 1 | 1 | Minus, South, West, Left, From, Below |
Sign/Status Matrix (BNR format)
| Bit 31 | Bit 30 | Meaning (BNR Numeric) |
|---|---|---|
| 0 | 0 | Failure Warning |
| 0 | 1 | No Computed Data (NCD) |
| 1 | 0 | Functional Test (FT) |
| 1 | 1 | Normal Operation |
Up to nineteen bits are usually used for the data and are mapped in the bits 29-11.
BCD data
Because the BCD value is encoded with four bits, five 'digits' can be fitted in the field. The MSB (bits 29-27) value range is 3 bits (0-7) and the others 4 bits (0-9)
For instance the value "75839" is encoded in binary as "111 0101 1000 0011 1001"
BNR data
With the BNR format, the field is coded in two's complement fractional notation. The sign of BNR Numeric Data words should be encoded in the SSM field at bit 29 (1=negative or 0=positive).
The Source/Destination Identifier (SDI) can be used to identify which source is transmitting the data or to identify which receiver the data is meant for.
This field is used in BCD/BNR mode but not available if alphabet ISO #5 is used.
As discussed before, the label (bits 1-8) contains a label word (in octal) indicating the data type (BNR, BCD, etc). The bits of the label are reversed.
Some equipment supports an alternate 25-bit word. The format is a simplified version of the 32-bit word and consists of an 8-bit label, a 16-bit data word and a parity bit.
| Word | Description | Usage |
|---|---|---|
| Bit 25 | Parity Bit | Parity Bit (for Integrity Check) |
| Bits 24-10 | Data | Data to be transmitted |
| Bits 8-1 | Label | Identify the data type; value in octal |
The fields have the same meaning than the 32-bit format described before.
Building / decoding ARINC 429 words is not simple with the tricky bit alignments within the word.
MXF4 provides helper functions used to facilitate the formatting / decoding of the ARINC 429 words.
Refer to the section Word Formatting for more details.
For more information on the ARINC 429 protocol, the complete specifications can be found in the following document:
429P1-18 Digital Information Transfer System (DITS), Part 1, Functional Description, Electrical Interfaces, Label Assignments and Word Formats
