3.2 ARINC 429 32-Bit Word Architecture

3.2 ARINC 429 32-Bit Word Architecture

Once the pair is electrically healthy, every parameter that travels on it is packaged as a 32-bit word. Practice on the former detailed data-bus scope commonly probes this map: which bits are the label, which way the label is numbered, what SDI and SSM are for, and whether parity is odd or even. Memorise the field boundaries first, then the encodings, then a handful of air-data labels so that an analyser display means something on the flight line.

Word positions are numbered 1 to 32. Transmission starts with the eight-bit label, whose most-significant label bit is sent first; after the label, SDI, data, SSM and parity proceed in increasing word-position order through bit 32. Host-interface documentation may store or display the label byte reversed, so distinguish the logical octal label from a component’s register representation.

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The five fields

There is no start bit, no stop bit and no separate CRC. The word gap (idle NULL) that precedes bit 1 is a timing feature, not a data field; it is covered in the next section.

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Bits 1–8: octal label, label MSB first

The label tells every sink which parameter this word is. Labels are written as three octal digits from 000₈ to 377₈ (8 bits cannot hold 400₈ or above). The octal notation exists because three digits map cleanly onto an 8-bit field whose most significant octal digit uses only two bits (values 0–3).

The label is the exception to the normal serial ordering: its most-significant label bit is transmitted first. The fields that follow the label are transmitted least-significant bit first. Some interface chips store the label byte in a reversed bit order, so a register dump may not look like the serial label unless the component documentation is followed.

Worked example — label 206₈ (a typical industry teaching label for indicated airspeed):

• 206₈ = 2×64 + 0×8 + 6 = 134 decimal.
• As a logical eight-bit label, 134 decimal is 10000110 binary.
• Because the label MSB goes first, the label portion appears on the wire as 1, 0, 0, 0, 0, 1, 1, 0 before SDI and data.

If an analyser supplies the first eight serial bits, read them as the label MSB-to-LSB sequence and convert the resulting byte to octal. If an interface register supplies a stored byte, apply that device’s documented label-bit convention before converting.

Typical industry teaching labels (air-data and attitude)

These octal labels are widely used in manufacturer characteristics and maintenance training. They are typical industry teaching examples, not numbers that EASA publishes as a syllabus table. Learn a short set so analyser traces are readable:

Sinks select by label. A display unit programmed to use 203₈ and 206₈ ignores a 314₈ heading word on the same bus. That is why one air-data source can feed many different consumers without addressing each consumer in the data field.

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Bits 9–10: Source/Destination Identifier (SDI)

Many aeroplanes carry more than one copy of the same sensor: left and right air-data computers, captain and first officer inertial units, or multiple radio receivers. Those units may emit the same octal label. The 2-bit SDI distinguishes up to three addressed installations, with a fourth code used as a common or unused state:

• 00 — SDI not used, or a common/all-call meaning defined by the characteristic.
• 01, 10, 11 — installation 1, 2 or 3 (for example left / right / centre), as assigned in the system design.

If a type needs more than four identities for the same parameter, the design allocates additional labels rather than stretching SDI. When you interpret an analyser, always read label and SDI together: 206₈ with SDI 01 is not the same stream as 206₈ with SDI 10.

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Bits 11–29: data — BNR, BCD and discrete

Nineteen bits are available for the payload. Unused bits are padded to 0. Three encodings appear in Module 5 teaching.

BNR (binary) words

A BNR word carries a numeric value in binary, typically as a two’s-complement signed number. Bit 29 is the sign bit (MSB of the data field): 0 for positive, 1 for negative in the usual two’s-complement reading. Bit 11 is the least significant data bit. The manufacturer’s ARINC characteristic states the range, the number of significant bits, and the LSB weight (resolution = full-scale range / 2^n).

Worked teaching example (not an official EASA scale): suppose indicated airspeed is encoded as BNR with a positive full-scale of 512 knots using 11 magnitude bits. LSB weight = 512 / 2^11 = 512 / 2048 = 0.25 knot. A magnitude field of 640 counts is 640 × 0.25 = 160 knots. The remaining data bits are pad zeros. The sink applies that scale only because its software knows label 206₈ is airspeed; the bits on the wire are just a binary integer plus sign.

BCD (binary-coded decimal) words

A BCD word packs decimal digits as 4-bit groups (0000–1001 for 0–9). This encoding is natural for parameters that humans already think of in decimal, such as a radio frequency (118.00–135.95 MHz class values) or a barometric setting in hectopascals. Illegal groups 1010–1111 are not decimal digits and indicate a defect or a different sub-field. BCD does not use two’s complement; sign or direction, when needed, is often carried in the SSM rather than in a data-field sign bit.

Discrete words

A discrete word uses selected bits in the data field as independent flags (gear down and locked, autopilot engaged, comparator warning, and so on). There is no single numeric scale. A sink must know which bit number means which condition. Mixing up a discrete word with a BNR scale produces nonsense engineering units on an analyser that has been given the wrong characteristic file.

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Bits 30–31: Sign/Status Matrix (SSM)

The SSM tells the sink whether the data field is usable and, for some word types, the sign or direction of the quantity. For BNR words the four codes map onto the four status meanings that Module 5 expects you to name:

Those four names — Normal Operation, Functional Test, No Computed Data, Failure Warning — are the examination set. A display that sees Failure Warning typically flags the parameter (amber or red, with a failure cue) rather than plotting a smoothly wrong needle. Functional Test must not be treated as live air data. NCD is not the same as Failure Warning: the box may be healthy but not yet producing a solution.

For BCD words the SSM is often used as a sign/direction code rather than as Failure Warning:

That is why a heading or latitude BCD-style word can reverse sign without stealing a data digit. Discrete-word SSM usage follows the characteristic; do not force the BNR table onto a discrete word without checking. In every case the sink must read SSM before trusting bits 11–29.

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Bit 32: odd parity

Bit 32 is set or cleared so that the total number of 1-bits in the 32-bit word is odd. That includes the parity bit itself. Even parity is wrong on ARINC 429 and is a standard distractor. A sink that detects even parity discards the word. Parity does not identify which bit flipped; it only declares that the word is damaged. When an analyser counts parity errors, look for a noise, loading, or wiring fault rather than a slightly wrong engineering-unit scale.

Compute parity last, after label, SDI, data and SSM are filled. Changing a data bit without recomputing bit 32 makes a legally encoded parameter look like a transmission error.

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Putting a word together

A complete teaching word might be: label 203₈ (pressure altitude), SDI 00, BNR altitude in bits 11–29, SSM 11 (Normal Operation), bit 32 chosen for odd parity. The same source can emit 206₈ on the next word, then 211₈, cycling through its label set at the refresh rates in the characteristic. Sinks need not receive every label; they decode only the labels they are programmed to use, provided SSM and parity are acceptable.

If you remember only one picture from this section, remember this order on the wire: label (label MSB first) → SDI → data → SSM → odd parity.