ARINC 429 Word Decoder
Decode a 429 word: octal label, SDI, SSM, BNR/BCD and odd parity.
Parity checks out. Read the other way round the label is 241 — sources number the label bits in opposite directions, so check which your tool means.
This tool decodes a 32-bit ARINC 429 word. Paste it as hex and it returns the octal label — under both bit-order conventions, because published sources disagree — plus the SDI, the SSM read for your data format, the BNR and BCD views of the data field, and an odd-parity check on bit 32. Reference: 0xE00640A1 decodes field by field with valid parity.
How it works
An ARINC 429 word is thirty-two bits with a fixed map: label, source or destination identifier, data, sign/status matrix, parity. Bit 1 leaves first, and the label leaves ahead of everything else. What makes the label awkward is that its bits run in the opposite direction to every other field, so writing the word as a hex number forces a choice about which end of the label byte is most significant — and the public documentation splits on that choice.
Rather than pick one and hope, this decoder treats it as a setting and always prints the other reading beside the answer. If your analyser says 205 and your ICD says 241, that is not a fault in either: they are the same eight bits counted from opposite ends. The same restraint governs the rest of the tool — the SSM is only interpreted once you say whether the parameter is BNR, BCD or discrete, and the BNR value stays raw until you supply the full-scale range that gives it units.
Worked example
P 1 · SSM 11 · data 0000000011001000000 · SDI 10 · label 10100001
parity valid (odd across all 32 bits)
label reads one way as 205, the other as 241
BNR 512 full scale → 268
Word fields
| Parameter | Value | Notes |
|---|---|---|
| Bits 1–8 | Label | octal, bit order contested |
| Bits 9–10 | SDI | source/destination |
| Bits 11–29 | Data | BNR or BCD |
| Bit 29 | BNR sign | 1 = negative |
| Bits 30–31 | SSM | meaning follows the format |
| Bit 32 | Parity | odd, over all 32 bits |
| BCD | 5 digits | top digit is 3 bits |
| Bit rate | 12.5 or 100 kbit/s |
Field map and parity cross-checked against the AIM ARINC 429 tutorial, the GE Intelligent Platforms / Condor-Ballard protocol tutorial, and the Holt HI-35850 datasheet. Label assignments and equipment IDs are deliberately not published here — they live in the paid specification and in per-aircraft interface control documents.
FAQ
▸Why does my label read 205 in one tool and 241 in another?
Because the two tools number the label bits in opposite directions, and the readings are bit-reverses of each other. Public sources genuinely disagree: some annotate word bit 1 as the label's most significant bit, others put the MSB at bit 8. Both describe the same sequence on the wire — the difference only appears once you write the word as a hex number. This decoder shows both readings on every word so you can match whichever your equipment documentation uses.
▸How is an ARINC 429 word laid out?
Thirty-two bits, numbered from 1: label in bits 1-8, SDI in bits 9-10, data in bits 11-29, SSM in bits 30-31 and odd parity in bit 32. Bit 1 goes out first, and the label is the one field whose bits are ordered opposite to the rest — which is exactly why its octal reading is contested.
▸What does the SSM mean?
It depends on the data format, which is why there is no single table. For BNR it reports failure warning, no computed data, functional test or normal operation; for BCD it carries sign and direction (plus/north/east versus minus/south/west) alongside the same failure states; discrete words use their own reading. Tell the tool which format your parameter uses and it shows that one, otherwise it shows the alternatives rather than guessing.
▸Why do I have to enter the full-scale range myself?
Because a BNR field carries no units. The 19 data bits are a signed two's-complement number whose meaning comes from the parameter's full-scale range, published in the equipment's interface control document. The decoder gives you the raw signed value and applies whatever scale you supply.
▸Why is there no table of what each label means?
Label assignments live in the ARINC 429 specification and in per-aircraft interface control documents, which are paid, controlled documents. Publishing a guessed table would produce silently wrong readings, so this tool reports the octal label and leaves the meaning to your ICD. The same applies to equipment IDs.
▸Is my data uploaded?
No. The word is decoded entirely in your browser.