Arinc429 Bus Loading

ARINC 429 Bus Loading (avionics/data-bus/arinc429-bus-loading)

Use when the task is budgeting an ARINC 429 transmit schedule for a civil avionics data bus: how many words per second the label rate table asks the transmitter to send, what that costs in bus occupancy, and whether the schedule fits the link with headroom. Every transmitted ARINC 429 word occupies 36 bit-times on the bus, 32 data bits plus the 4-bit gap between words, so a schedule of label rates sums into a total word rate and the word rate prices the bus directly at 36 bits per word. The module is pure Python stdlib and deterministic; you supply the label rate schedule (a list of rates in labels per second, or a dict mapping a label to its rate) and it returns the load, utilization, capacity verdict and design headroom. It pairs with avionics/data-bus/arinc429-protocol, which covers the 32-bit word layout and parameter coding itself (this leaf never builds or interprets a word), and with avionics/data-bus/arinc664-afdx and mil-std-1553 for the other civil and military bus loading models.

Domain quick reference

• Every ARINC 429 word occupies BITS_PER_WORD = 36.0 bit-times (32 data bits plus a 4-bit gap), whatever the label content, so the word rate alone determines bus occupancy.
• Total word rate: total_word_rate(label_rates) = sum of the per-label rates. A schedule of 3 labels at 50 Hz, 4 at 20 Hz and 2 at 10 Hz totals 350 + 420 + 2*10 = 250 words/s.
• Bus load: bus_load_bps(words_per_s) = words_per_s * 36. The 250 words/s schedule loads 250 * 36 = 9000 bps.
• Percent utilization: percent_utilization(load, link_rate) = load / link_rate * 100. Against the 100 kbps link (RATE_100_KBPS = 100000.0) the 9000 bps load is 9.0 percent.
• Word capacity: word_capacity(link_rate) = link_rate / 36. The 100 kbps link carries 100000 / 36 = 2777.8 words/s; the 12.5 kbps link (RATE_12_5_KBPS = 12500.0) carries 347.2 words/s.
• Verdict rule: OVER when the utilization exceeds 100 percent, FITS otherwise. A 3000 words/s schedule at 100 kbps loads 108000 bps, 108.0 percent, so it needs a second bus or a rate reduction.
• Headroom: headroom = max(0, DESIGN_GUIDELINE_PCT - utilization) with DESIGN_GUIDELINE_PCT = 80.0, the common design guideline that keeps about 20 percent of the bus free for growth.
• The identity: a schedule at exactly the word capacity sits at 100 percent utilization; at exactly the 80 percent design load the headroom is zero; doubling every label rate doubles the utilization.
• ARINC 429 (Mark 33 DITS) frames the bus context; the relations above are standard engineering methodology, summary-only.

Workflow

1. Collect the transmit schedule: every label on the bus and its transmission rate in labels per second (a list of rates, or a dict {label: rate}).
2. Sum the schedule with total_word_rate to get the total words per second the transmitter must emit.
3. Price the bus with bus_load_bps (words per second times 36 bits per word) to get the load in bits per second.
4. Normalize to the link with percent_utilization, using the default RATE_100_KBPS or the explicit RATE_12_5_KBPS low-speed link.
5. Get the capacity with word_capacity and the verdict dict with capacity_verdict(total_words_per_s): capacity_wps, utilization_pct, verdict (OVER when utilization exceeds 100 percent, else FITS) and headroom_pct against the 80 percent guideline.
6. For the full budget in one call, run bus_loading_summary(label_rates) and read total_words_per_s, load_bps, utilization_pct, capacity_wps, verdict and headroom_pct.
7. If the verdict is OVER, cut label rates, merge low-rate labels, or split the schedule across a second bus, then re-run the summary.
8. Confirm the deterministic checks with the contract test scripts/test_arinc429_bus_loading.py.

Worked example

Reference schedule on the 100 kbps link: 3 labels at 50 Hz, 4 labels at 20 Hz, 2 labels at 10 Hz.

• total_word_rate: 350 + 420 + 2*10 = 250.0 words/s.
• bus_load_bps: 250.0 * 36.0 = 9000.0 bps.
• percent_utilization: 9000.0 / 100000.0 * 100 = 9.0 percent.
• word_capacity: 100000.0 / 36.0 = 2777.78 words/s.
• capacity_verdict(250.0): utilization 9.0 percent, verdict FITS, headroom 80 - 9 = 71.0 percent. The schedule uses well under the guideline.
• Adding one 100 Hz label: 350.0 words/s, 12600.0 bps, 12.6 percent, still FITS.
• Over-capacity check: 30 labels at 100 Hz total 3000.0 words/s, 108000.0 bps, 108.0 percent utilization, verdict OVER, headroom 0.0: the schedule needs a second bus or a rate reduction.
• Low-speed link at 12.5 kbps: capacity 12500.0 / 36.0 = 347.2 words/s; a 300 words/s schedule is 86.4 percent (FITS, but above the guideline so headroom is 0.0), and 350 words/s is 100.8 percent, OVER.

Verification

• Confirm total_word_rate of the reference schedule is 250.0 words/s and of a single 1 Hz label is 1.0 word/s.
• Confirm bus_load_bps(250.0) is 9000.0 bps and bus_load_bps(100.0) is 3600.0 bps.
• Confirm percent_utilization at the word capacity is 100.0 percent within 1e-6, and 108.0 percent for 3000 words/s.
• Confirm capacity_verdict(300.0, RATE_12_5_KBPS) gives 86.4 percent FITS and capacity_verdict(350.0, RATE_12_5_KBPS) gives 100.8 percent OVER.
• Confirm the headroom is 71.0 at the 9 percent reference load, 0.0 at the 80 percent design load and 0.0 over capacity.
• Confirm bus_loading_summary returns exactly the documented keys.
• Confirm ValueError on an empty schedule, a negative rate, a non-positive link rate and non-positive bits per word.
• Run the contract test offline: python3 scripts/test_arinc429_bus_loading.py (34 tests, deterministic).

Related leaves

• avionics/data-bus/arinc429-protocol: the companion leaf for the 32-bit word layout and parameter coding of the words this leaf schedules; it states the 36 bit-time occupancy fact but does not sum a per-label schedule or compute utilization.
• avionics/data-bus/arinc664-afdx: the switched network companion leaf whose 100 Mbps link utilization budget is the symmetry anchor for this utilization calculation.
• avionics/data-bus/mil-std-1553: the military multiplex bus scheduling companion with its 1 Mbps dual-redundant bus and message timing model.

Pitfalls

• Pricing a schedule at 32 bits per word: every transmitted word occupies BITS_PER_WORD = 36.0 bit-times (32 data bits plus the 4-bit gap) and bus_load_bps multiplies by 36; pricing at 32 understates the load by about 12.5 percent and can hide an OVER verdict.
• Treating the 80 percent guideline as the verdict threshold: the verdict flips to OVER only when utilization exceeds 100 percent, so a schedule at 86.4 percent on the 12.5 kbps link is FITS with headroom 0.0 - zero headroom is not the same as OVER.
• Forgetting the low-speed link: at RATE_12_5_KBPS the capacity is 12500.0 / 36.0 = 347.2 words/s, so a 350 words/s schedule is already 100.8 percent and OVER even though it fits the 100 kbps link - carry the link rate into the capacity check.
• Summing bit rates instead of word rates: total_word_rate adds the per-label rates in words per second and only bus_load_bps prices them at 36 bits each, so never multiply the label rates by 36 twice.
• Routing word-format questions here: this leaf never builds or interprets a 32-bit word - the label/word layout and parameter coding live in avionics/data-bus/arinc429-protocol; feed this leaf the label rate schedule only.
• Feeding an empty schedule or a negative rate: the summary and capacity functions raise ValueError on an empty schedule, negative rates, non-positive link rates, and non-positive bits per word - validate the schedule before calling.

Behavior contract (gate 3)

Run the deterministic contract test (stdlib unittest, offline):

python3 scripts/test_arinc429_bus_loading.py

The test covers the reference schedule totals and bounds, the 36 bits-per-word load pricing, percent utilization at and beyond the word-per-second capacity, the 12.5 kbps link capacity and verdicts, the headroom boundaries at the 80 percent guideline, the bus_loading_summary convenience dict keys and values, the doubling and determinism identities, the module constants, and ValueError rejection of an empty schedule, negative rates, non-positive link rates and non-positive bits per word.

Compliance

• Standards referenced, not reproduced: ARINC 429 (Mark 33 DITS) is the ARINC standard for civil avionics point-to-point buses; the bus loading relations above are standard engineering methodology, summary-only per standards-map.yaml.
• compliance: STANDARDS-REF, gated: false.