# Arinc429 Protocol > Use when checking a bus monitor decode, writing a test stimulus for an LRU interface, or explaining the one-transmitter up-to-20-receivers twisted shielded pair topology. Encode and decode ARINC 429 digital information transfer words for avionics data buses: build the 32-bit word from the octal label, SDI, 19-bit data field, SSM, and odd parity bit, split a received word back into its fields, and convert BNR and BCD parameters to and from engineering units at the word rate of 12.5 or 100 kbps. Trigger: ARINC 429, data word, octal label, SDI, SSM, parity, BNR, BCD, 100 kbps, 12.5 kbps, avionics data bus. - Skill: `ashfordeou/arinc429-protocol` (Agent Skill, multi-file: 3 files) - Install (CLI): `npx skillmds@latest add ashfordeou/arinc429-protocol` - Raw SKILL.md: https://api.skillmd.com/api/skills/ashfordeou/arinc429-protocol/raw - Safety review: pending - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: Docs & Writing - License: Apache-2.0 - Author: ashfordeOU (https://skillmd.com/u/ashfordeou) - Updated: 2026-09-17 - Page: https://skillmd.com/skills/ashfordeou/arinc429-protocol --- # ARINC 429 Data Bus Protocol (avionics/data-bus/arinc429-protocol) Use when the task is the ARINC 429 digital information transfer standard (Mark 33 DITS) for civil avionics: the 32-bit word format and bit numbering, octal label addressing and common label tables, word transmission at 12.5 or 100 kbps over a twisted shielded pair, the one-transmitter up-to-20-receivers topology, and encoding and decoding of typical parameters in BNR (binary) and BCD formats. The module is data-driven: you supply the label table and coding choices from the applicable equipment definition, and the functions validate bounds, pack and unpack words, compute odd parity, and convert data fields to and from engineering units. ## Domain quick reference - ARINC 429 transfers 32-bit digital words point-to-point over a twisted shielded pair at 12.5 or 100 kbps, with exactly one transmitter driving up to 20 receivers on a bus. - Word layout, ARINC bit numbering (bit 1 is the least significant bit and is transmitted first; in the integer, bit 0 = ARINC bit 1): label bits 1-8 (octal), SDI bits 9-10, data bits 11-29 (19 bits), SSM bits 30-31, parity bit 32. - Octal labels: label 010 octal = 8 decimal; the label field spans 000 to 377 octal. Label-to-parameter assignments come from ARINC 429 appendix tables and the equipment definition; confirm the applicable table before mapping. - Worked encode anchor: label 010, SDI 1, data field 1234, SSM 3 (normal operation) -> word 0x60134908 (1611876616 decimal) with odd parity. - Odd parity: the full 32-bit word always has an odd number of 1 bits. The anchor payload (bits 1-31) has 9 set bits, so parity bit 32 = 0 and the word stays odd. - BNR: two's complement, with the data MSB (bit 29) as the sign bit and the least significant bit weight equal to the scale factor. Worked: 123.4 at scale 0.1 -> field 1234; -12.3 at scale 0.1 -> field 524165 (two's complement 19-bit), decoding back to -12.3. - BCD: four decimal digits in the low 16 data bits, plus an optional fifth digit (0-7) in the top 3 bits. Worked: 1234 -> field 4660. - SSM conventions (typical): 11 normal operation, 10 functional test, 01 no computed data, 00 failure warning. Interpretation can differ between BNR and BCD words; confirm per label. - Word timing: 32 data bit-times plus a 4-bit gap between words -> about 2778 words per second at 100 kbps, about 347 words per second at 12.5 kbps. - The exact word-format tables, label tables, and equipment identification tables are revision-specific standard data; confirm them against the current revision before freezing an interface design. ## Workflow 1. Identify the bus direction, the word rate (12.5 or 100 kbps), and the parameter to transfer; look up its octal label and data format (BNR or BCD) in the applicable equipment table. 2. Convert the engineering value to the raw 19-bit data field with bnr_encode (scale factor for the LSB weight, two's complement) or bcd_encode (decimal digits). 3. Pack the word with build_word(label, sdi, data, ssm); the odd parity bit is computed and placed in bit 32 automatically. 4. On reception, split the word with decode_word and check parity_ok before trusting the payload; a parity failure flags a corrupted or marginal transmission. 5. Convert the data field back to an engineering value with bnr_decode or bcd_decode, and interpret the SSM (normal operation vs failure warning, no computed data, or functional test). 6. Verify the transmit budget: word rate at the chosen speed, and the receiver count (up to 20) against the single transmitter. 7. Report the encoded or decoded fields, the parity verdict, and any label table entries that still need confirmation against the current revision. ## Pitfalls - Confusing the ARINC 429 data bus with the aircraft electrical bus in do160/power-input: power-input is section 16 equipment power characteristics (steady-state voltages, sag/surge transients); 429 is the digital data transfer protocol. A voltage test does not touch word format, and a label decode does not touch voltage limits. - Confusing this leaf with do160/radio-frequency-susceptibility: RS103 radiated and CS114 conducted immunity tests concern RF interference on equipment and wiring; this leaf concerns how digital words are formatted, coded, and transferred, not immunity levels. - Confusing this leaf with flight-management/flight-planning: FMS route tasks (waypoints, great-circle track, leg geometry) operate on the navigation content that may ride an ARINC 429 bus; 429 is the transfer protocol, not the navigation math. - Confusing this leaf with far-cs25/special-conditions: special conditions address novel design features for the certification basis; 429 is a wiring and interface standard, not a certification topic. - Bit-order mistakes: ARINC bit 1 is the LSB and is transmitted first, so the label sits in the LOW byte of the integer, not the high byte. - Octal label traps: label 010 octal is 8, not 10 decimal; passing 10 decimal encodes label 012, a different word. - Parity is odd, not even: the 32-bit word must contain an odd number of 1 bits; recompute the parity bit whenever the payload changes. - Mixing BNR and BCD: BNR is signed two's complement scaled by the LSB weight, BCD is packed decimal digits; the SSM meaning can also differ between the two formats. - Forgetting the one-transmitter rule: two transmitters on one bus is a wiring violation; up to 20 receivers are permitted on the pair. - Treating the label table as fixed: label-to-parameter assignments are standard data that vary by equipment definition; confirm every label against the applicable table before freezing the interface. ## Behavior contract (gate 3) The logic is exercised by the gate 3 contract test: scripts/test_arinc429_logic.py against scripts/arinc429_logic.py (stdlib unittest, offline). Run: python3 scripts/test_arinc429_logic.py ## Compliance - Standards referenced, not reproduced: ARINC 429 text is proprietary (ARINC/SAE ITC); summary-only per standards-map.yaml. - The module implements the bit layout, odd parity, and BNR/BCD coding helpers from common knowledge; no standard table is embedded in the code or this page. - compliance: STANDARDS-REF, gated: false.