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MX Foundation 4
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MX Foundation 4
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ARINC 629, is a specification for a bus on which terminals are connected. There is no controller on the bus. Each terminal on the bus is able to identify when it is its turn to transmit. Synchronization between the terminals is ensured by three timers (the transmit interval, the synchronization gap and the terminal gap).
Up to 120 terminals can transmit on one bus. ARINC 629 messages are 31 wordstring long (maximum). Word strings is a 20-bit long label word followed by 0 to 256 16-bit long data words. Messages are transmitted at a speed of 2Mbps.
At the physical layer, it features communication of messages over bidirectional, unshielded, bipolar electrical interface. Bit-encoding is Manchester encoded doublets.
Data is transmitted in groups called Messages. Messsages can have different lengths. After each message, there is a unique Terminal Gap (TG). Messages are composed of a maximum of 31 wordstrings that can also have variable lenghts. Between each wordstring, there is a 4 bit time gap. Wordstrings are a label word followed by 0 to 256 data words. Words (label words and data words) are 20 bit long. Before the first label word of a message comes a one half bit time pulse called pre sync sync pulse (PSSP).
A label word is defined as follows:
| Word | Description | Usage |
|---|---|---|
| Bits 1-3 | Synchronization | Sync pulse (PS) is a high-to-low pulse. First 1.5 bit is high, last 1.5 bit is low. |
| Bits 4-15 | Label field | Uniquely identify the parameters which are transmitted in each data word of the wordstring. |
| Bits 16-19 | Label extension field | See CID |
| Bit 20 | Parity | Word parity is ODD and includes the sync pattern as a "data bit". |
A data word is defined as follows:
| Word | Description | Usage |
|---|---|---|
| Bits 1-3 | Synchronization | Sync pulse (PS) is a low-to-high pulse. First 1.5 bit is low, last 1.5 bit is high. |
| Bits 4-19 | Data field | See Data Types |
| Bit 20 | Parity | Word parity is ODD and includes the sync pattern as a "data bit". |
At the physical layer the data words are transmitted on the data bus the synchronization bits first and then the data (depends upon word type) followed by the parity bit. The data is transmitted with LSB first and MSB last.
Typically the data types used by ARINC 629 are:
Binary Number Representation (BNR): In fractional binary two's complement.
BNR encoding stores data as a binary number. BNR is most efficient in terms of bus capacity and should generally be used. The sign of BNR Numeric Data words should be encoded on bit 19. Negative BNR numbers are encoded as the two's complements of positive values with the negative sign denoted as a logic 1 in data bit 19.
Binary Code Decimal (BCD): Numerical Subset of ISO Alphabet No 5
BCD encodes each decimal value in 4-bit digit.
For instance the value "75839" is encoded in binary as "111 0101 1000 0011 1001"
and other less used data types:
| X | |||||
|---|---|---|---|---|---|
| Y | 1 | 2 | 3 | 4 | |
| 1 | A | B | C | D | |
| 2 | E | ||||
| 3 | F | G | |||
| X | ||||
|---|---|---|---|---|
| Y | 1 | 2 | 3 | |
| 1 | A | B | C | |
| 2 | D | E | ||
| 3 | F | |||
When several identical LRUs have the same part numbers and use the same label, the CID helps make the differentiation between them. The CID is a four bit identification that can accommodate up to sixteen LRUs with the same part number. The CID is sent in the label word on the label extension bits (label bits 16 to 19).
The label is transmitted in the label word on bits 4 to 15. The label uniquely identify the parameters which are transmitted in each data word of the wordstring. They are listed in ARINC Specification Part 3 - Data Standards.
Due to the absence of actual bus controller, there are three timers that allow each RT to transmit when it should and avoid any data collision on the bus. Two timers are global (transmit interval and synchronization gap) and one is specific to each RT (terminal gap).
A RT should transmit when all three of its timers have elapsed.
The transmit interval is a global parameter. It is the longest of the three timers. Once a RT has transmitted, it has to wait (at least) the length of time specified by the TI before being able to transmit again. It ranges from 0.5005625 to 64.0005625 milliseconds which corresponds to 1,000+ to 128,000+ bit times at the transmit rate of 2Mbps. Seven bits specify the binary value (BV) of the TI. The TI length of time in milliseconds is the decimal value (BV in base 10) divided by 2 plus 0.5005625. The TI starts when the RT starts transmitting.
Example: BV is 0000010 or, in base 10, 2. Therefore, TI = 2/2 + 0.5005625 = 1.5005625 ms
The synchronization gap is the second longest timer and, like the TI, is set to the same value for all RT. The SG is chosen to be longer than the TG and its binary value has to be 16, 32, 64 or 127. These values correspond to 17.7 μs, 33.7 μs, 65.7 μs and 128.7 μs. The SG timers starts as soon as a transmission stops (i.e. when bus is quiet). If a transmission occurs on the bus before SG has elapsed, SG is reset. Once the SG has elapsed, the SG is not reset if a transmission occurs on the bus. The SG will be reset when its RT starts transmitting.
The terminal gap is unique to each RT. RTs should be assigned a unique number N from 2 to 126. Several RTs should not have the same number N. The TG length of time in microsecond is N + 1.6875.
Example: Third RT is N = 3. Its TG should be 4.6875 μs.
TG begins right after the SG has elapsed. However, unlike the SG, the TG is reset when a transmission occurs, no matter if the SG had elapsed or not.
For more information on the ARINC 629 protocol, the complete specifications can be found in the following document: Multi-Transmitter Data Bus Part I Technical Description - ARINC Specification 629P1-5
