// DECODER
Paste a 32-bit word as your bus analyser or interface card shows it, in hexadecimal. The decoder lays it out bit by bit: the label in octal, the SDI, the sign/status matrix, the parity check, and the data field read the way the label's definition says — binary (BNR) with its range, binary-coded decimal (BCD) or discrete bits.
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Label (octal)
203
Read the other way: 301
Value (BNR)
10,000
Normal operation
The word, bit 32 to bit 1
| Word | |
|---|---|
| Hexadecimal | E138 80C1 |
| Parity (bit 32) | Odd: correct |
| Fields | |
| Label (bits 1–8) | 203 octal · 83 hex · byte in the word C1 |
| SDI (bits 9–10) | 00 (0) |
| SSM (bits 30–31) | 11 · Normal operation |
| Data field (bits 11–29) | 04E20 hex |
| Read as BNR | |
| Sign (bit 29) | 0: positive |
| Resolution | 1 (range / 2^17) |
| Values the word can carry | −131,072 to 131,071 |
| Bit 11, below the significant bits | All zero |
| Read as BCD | |
| Digits, bit 29 down to bit 11 | |
| Every digit from 0 to 9 | |
| Read as discrete bits | |
| Bits set (11 to 29) | |
Check the highlighted field: every box needs a valid number.
HOW IT IS COMPUTED
ARINC 429 numbers the bits 1 to 32 in the order they are sent, and the diagrams of the standard draw bit 32 on the left. The decoder reads the word the same way: the first hexadecimal digit holds bits 32 to 29.
P (32) · SSM (31–30) · data (29–11) · SDI (10–9) · label (8–1)
Nineteen data bits between the two bits of the source/destination identifier and the two of the sign/status matrix, and a parity bit set so that the word holds an odd number of ones.
label 213 (octal) = 8B hex → D1 in bits 1–8
The data bits are sent least significant first, the label most significant first. Read with bit 1 as the least significant bit, the label byte therefore holds the label reversed. Many interface cards reverse it back on reception: the switch above says which kind wrote your word.
value = N × range / 2^s
N is the two's-complement number formed by the sign in bit 29 and the s significant bits from bit 28 down. Bit 28 weighs half the range, and the resolution is the range divided by 2^s. Bits below the significant ones are padding, or discretes the label defines.
D1 (29–27) · D2 (26–23) · D3 (22–19) · D4 (18–15) · D5 (14–11)
Five decimal digits, the first on three bits. For BCD the sign/status matrix also carries the sign: 00 for plus, north, east, right, to or above; 11 for minus, south, west, left, from or below. Labels with fewer digits use the low bits for discretes.
BNR: 11 normal · 00 failure warning · 01 no computed data · 10 functional test
The table depends on the format: for discrete words, 00 is verified data and 11 failure warning. It is the field to read before believing the number.
What a label carries, its format, range and resolution come from the definition of that label on that equipment: the decoder only applies them. The example is a BNR word with label 203, a range of 131,072 and 17 significant bits, so a resolution of 1: it carries 10,000.
Further reading
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