ARINC 429¶

A repeated 32-bit avionics word becomes meaningful only in label context.

A Concrete Artifact¶

# synthetic ARINC 429 word rendered as an integer
0xA4102F8B

Synthetic offline sample for explanation; not a real operational trace or live-system instruction.

Inspection Trap¶

A single ARINC 429 word can be field-decoded before it is understood. The trap is to treat the label byte as an aircraft parameter without the LRU, installation, label-table, and representation authority that make that label meaningful.

This page inspects one observed synthetic 32-bit word as evidence: label, SDI, data, SSM, parity, cadence, and wrong-authority risk. It does not teach aircraft-specific label-table decoding or operational avionics interpretation.

Worked Decode¶

1. Treat 0xA4102F8B as one synthetic ARINC 429 word in the current integer-oriented teaching model.
2. The low-order byte is 0x8B; displayed as a simple byte value this is octal 213. In avionics work, labels are usually discussed in octal, while real ARINC tooling may expose label bit-order conventions at the wire/tool boundary.
3. Under the page-local field extraction, SDI = 3, data = 0x1040B, SSM = 1, and the parity bit is 1.
4. The word has odd parity in this synthetic representation, which is word-integrity evidence only.
5. A label is not yet an engineering value. BNR, BCD, discrete, SSM meaning, label assignment, LRU identity, and installation context are separate semantic authority.
6. A label table from the wrong equipment class can make the same label look like a credible altitude, speed, or discrete state. That is a plausible false value, not a decode success.

What The Evidence Supports¶

• The artifact supports a page-local field breakdown: label 0x8B, SDI, data bits, SSM, and parity.
• The parity bit supports word-integrity reasoning for the synthetic representation.
• Repeated labeled words can support cadence, validity transitions, stale/missing evidence, subsystem disagreement hypotheses, degraded observed communication/validity evidence, and repeated structure of labeled words when timestamped evidence is present.

What The Evidence Does Not Support¶

• The artifact does not identify an aircraft parameter, LRU, safety effect, or engineering unit by itself.
• It does not settle BNR, BCD, discrete, SSM, SDI, or label-table conventions without source authority.
• It does not model complete ARINC electrical behavior, wire-order treatment, or installation-specific label assignments.

Field Layout / Anatomy¶

Visual Model¶

Timing And Authority¶

Bus authority is transmitter ownership: one source drives a channel, receivers listen, and there is no arbitration among concurrent transmitters on the same pair. Semantic authority comes from ARINC label assignments plus aircraft, LRU, and installation context. BNR versus BCD, SSM interpretation, and SDI use cannot be recovered from the word alone.

Bytes rarely explain themselves. Name the source that defines meaning before naming engineering values: standard metadata, label table, DBC, PGN/SPN definition, object dictionary, register map, DID catalog, LDF, ARXML, channel metadata, or vendor profile.

Failure And Ambiguity¶

• A label from one equipment class may mean something different in another installation.
• BNR/BCD/discrete confusion can produce plausible false values.
• Ignoring SSM can turn failure-warning or no-computed-data words into confident measurements.
• A repeated label can be stale, latched, or rate-limited; cadence is validity evidence, not proof.
• Integer diagrams can hide wire-order and label-bit-order conventions; the local teaching model names that simplification.

Python Model¶

The current package has a concrete Arinc429Word model:

"""Runnable ARINC 429 word example for the schoolbus binder."""

from schoolbus.protocols.arinc429 import Arinc429Word

word = Arinc429Word.from_int(0xA4102F8B)

print(word.show_fields())
print(word.explain())

It teaches word inspection, not aircraft-specific label-table decoding.

The companion assessment example shows explicit label display convention, SSM representation context, BNR/discrete teaching decode, and repeated-word cadence evidence:

"""Runnable ARINC 429 assessment example for the schoolbus binder."""

from datetime import UTC, datetime, timedelta

from schoolbus.protocols.arinc429 import (
    Arinc429Representation,
    Arinc429Word,
    Arinc429WordObservation,
    assess_repeated_words,
    assess_ssm,
    decode_bnr_teaching,
    extract_discrete_bits,
    format_label,
)

word = Arinc429Word.from_int(0xA4102F8B)

print(format_label(word.label, convention="octal"))
print(format_label(word.label, convention="reversed-octal"))
print(
    assess_ssm(
        word.ssm,
        representation=Arinc429Representation.BNR,
        source="synthetic-profile",
    ).explain()
)
print(
    decode_bnr_teaching(
        word.data,
        bit_width=19,
        scale=0.125,
        units="units",
        source="synthetic-profile",
    ).explain()
)
print(
    extract_discrete_bits(
        word.data,
        bit_names={0: "teaching-bit-0", 3: "teaching-bit-3"},
        source="synthetic-profile",
    ).explain()
)

now = datetime(2026, 5, 21, 12, 0, tzinfo=UTC)
assessment = assess_repeated_words(
    (
        Arinc429WordObservation(word=word, observed_at=now - timedelta(milliseconds=20)),
        Arinc429WordObservation(word=word, observed_at=now),
    ),
    label=word.label,
    sdi=word.sdi,
    max_gap=timedelta(milliseconds=100),
    now=now,
)
print(assessment.explain())

The first workbook sample pack includes a passive synthetic ARINC 429 fixture under samples/first-workbook/ for label, SDI, data, SSM, and parity inspection with teaching-simplified display conventions.

The avionics workbook adds repeated-label observations from distinct synthetic authorities under samples/avionics-workbook/ for cadence, stale/missing, and bounded near-contemporaneous conflict evidence. Ordinary same-source value evolution is not labeled a conflict.

Source Notes¶

Source confidence: High for word anatomy and observer framing; label-specific engineering semantics require aircraft or LRU authority and are not redistributed here.

The 0xA4102F8B word is synthetic. The packet diagram is a compact field view, not a full treatment of ARINC bit numbering, electrical signaling, word gaps, or complete label-table behavior.

References¶

Public Sources¶

• SAE ITC / ARINC
• ARINC 429 Digital Information Transfer System — canonical-standard.
• UEI
• ARINC 429 tutorial — vendor-tutorial.

Project posture is aggregated in the protocol support policy, source policy, and project charter.