ARINC 429 Avionic Data Bus

Fig.1 - Example of ARINC 429 Bus Topology including flight test data recorder.

Cabling and bit representation:

A unidirectional ARINC 429 data bus requires a shielded 75 ohm twisted pair cable, grounded at both ends. Receiving devices should be connected in parallel with the transmitter. A second twisted pair can be used if bi-directional communication is required.

Over this twisted pair, a tri-state modulation method is employed to help provide an interference free and fail safe transmission. In this bipolar, Return-to-Zero (RZ), format, a binary 1 is indicated by the signal going from NULL state (0 v) to a HIGH state (nominally +5 v) for the first half of the bit, before returning to the NULL state for the remaining half of the bit cycle.

A binary 0 is indicated by dropping from NULL state (0 v) to LOW state (nominally –5 v) for the first half of the bit before returning to the NULL state for the remaining half of the bit cycle.

N.B. The standard defines a 10 volt differential between HIGH and LOW state, also known as Data A and Data B levels which is how you will often see ARINC terminals labels on receiving devices. There is a tolerance to allow for the dropping voltage levels as the number of receiving devices increases from one to twenty. The standard will also define a tolerance for the rise and fall times of the signal.

Bits are transmitted at either low speed (12.5 kbit/s), or high speed (100 kbit/s). For those of you who crave the clarity of Boolean logic, that’s an exclusive OR, i.e. only one speed should be used on a single data bus.

Return-to-Zero (RZ) representation of binary
Binary		1	0	1	0	1	1	0	0
Return-to-Zero representation	HIGH NULL LOW

 

A collection of 32 RZ coded bits become a word, and these carry the data across the bus in what ARINC refer to as Messages. Most Messages are 32 bit in length. When the transmitting device has no messages left to transmit, the bus returns to a NULL (0 v) state.

ARINC 429 Word Format:

Each ARINC word is a 32-bit value that contains five fields; the Label, SDI, the Data, SSM, and a final parity bit.

The diagram below shows the first transmitted bit on the right, the last on the left.

ARINC 429 Word Format
P	SSM		LSB		Data															MSB		SDI		LSB		Label				MSB
32	31	30	29	28	27	26	25	24	23	22	21	20	19	18	17	16	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1

 

We can rotate this table through 90° and expand it to include a full description. For clarity were reversing the order of the bits.

ARINC 429 Word Format, Expanded
Section	Bit	Description
Label	1	Identifies the data with an octal number in the range 000 to 377, allowing up to 255 Information Identifiers used in a look up table. We’ll talk more about this later on.   ARINC Label Structure Bit 8 7 6 5 4 3 2 1 Octal Units 2s 1s 4s 2s 1s 4s 2s 1s Decimal Range 0 to 3 0 to 7 0 to 7
	2
	3
	4
	5
	6
	7
	8
Source Destination Identifier	9	Optional, the SDI is used to signify the data source to the receiving device. If SDI is not used, these two bits can instead be used to increase the resolution of the data. ARINC SDI Matrix Structure Bit 9 Bit 10 System 0 0 All Systems 0 1 Systems 1 1 0 Systems 2 1 1 Systems 3
	10
Data	11	Commonly formatted as Discrete Data, Binary Coded Decimal (BCD), two's complement binary encoding (BNR), or a mixture. When BNR encoding is used bit 29 takes on special significance.   Sign matrix for BNR Data Bit 29 Definition 0 Plus, North, East, Right, To, Above 1 Minus, South, West, Left, From, Below
	12
	13
	14
	...
	26
	27
	28
	29
Sign / Status Matrix	30	Indicates a negative or positive value, direction, validity, and testing depending on data type and formatting used. The following matrix provide definitions.   Sign / Status matrix for BCD Data Bit 30 Bit 31 Definition 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 for BNR Data Bit 30 Bit 31 Definition 0 0 Failure Warning (FW) 0 1 No Computed Data (NCD) 1 0 Functional Test (FT) 1 1 Normal Operation (NO)   Sign / Status matrix for Discrete Data Bit 30 Bit 31 Definition 0 0 Verified Data, Normal Operation 0 1 No Computed Data (NCD) 1 0 Functional Test (FT) 1 1 Failure Warning (FW)
	31
Partity	32	Used for detecting errors in transmission. A receiver will expect an odd number of 1 bits in the word, and the parity bits is set to make this so.

More about Labels:

ARINC 429 data labels are octal numbers in the range 000 to 377, allowing for up to 255 Information Identifiers (for those new to Octal, there are no eights nor nines). These Information Identifiers act as a key in the index of a look up table. In fact there are multiple look up tables, typically one for each type of ARINC 429 transmitting equipment; Air Data Inertial Reference Unit, Flight Management Computer, radar, radio, GPS unit, etc.

A look up table should be provided when purchasing equipment that transmits IRIG 429, and will typically include information of data format (Discrete, BCD, or BNR), range, units, etc. Tables can also be found in the ARINC 429 standard documentation, but here is a flavour.

 

Example ARINC Label Table Data
Label (Octal)	Description	Format	Units	Range	Bits
102	Select Altitude	BNR	feet	0 to 65535	12
210	True Airspeed	BNR	kt	0 to 511 / 1023	10
212	Barometric Altitude Rate	BNR	fpm	0 to 255 / 8191	9

 

Groups of similar equipment tend to use common look up tables with minor manufacture’s variations in data format. This allows some degree of interchangeability of parts.