# Mil Std 1553 Bus Loading > Use when you must compute the MIL-STD-1553 bus loading: convert a minor-frame message schedule into wire-word counts per message type (command and status overhead plus data words), apply the fixed 24 microsecond word slot at the 1 Mbps data rate, sum the schedule time, and return the bus utilization against the minor-frame length with an 80 percent loading guideline verdict. Produces per-message wire words and time, the schedule total, the percent utilization, the headroom to the 80 percent budget, and a FITS or OVER verdict. Trigger: mil std 1553 bus loading, minor frame schedule, wire word count, bus utilization, data bus load, bc to rt, rt to bc, rt to rt, message overhead, loading headroom. - Skill: `ashfordeou/mil-std-1553-bus-loading` (Agent Skill, multi-file: 3 files) - Install (CLI): `npx skillmds@latest add ashfordeou/mil-std-1553-bus-loading` - Raw SKILL.md: https://api.skillmd.com/api/skills/ashfordeou/mil-std-1553-bus-loading/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/mil-std-1553-bus-loading --- # MIL-STD-1553 Bus Loading (avionics/data-bus/mil-std-1553-bus-loading) Use when the task is MIL-STD-1553 bus loading at the schedule level: how many wire words each message type puts on the 1 Mbps bus, how long the message occupies the bus, and whether the whole minor-frame schedule fits an 80 percent loading guideline. The model converts each message into command and status overhead plus data words, charges a fixed 24 microsecond word slot per wire word, and returns the utilization and the headroom to the budget. It pairs with avionics/data-bus/mil-std-1553, which owns the 20-bit word encode/decode and format classification side, and with avionics/data-bus/arinc429-bus-loading, the per-channel sibling loading model. ## Domain quick reference - Wire words per message (command and status overhead plus data words): BC-to-RT: 1 command + data_words + 1 status = data_words + 2; RT-to-BC: 1 command + 1 status + data_words = data_words + 2; RT-to-RT: 2 commands + 1 status + data_words = data_words + 3. A message carries 1 to 32 data words. - Word slot: WORD_TIME_US = 20.0 us (20 bit times per word at the 1 Mbps data rate) plus WORD_GAP_US = 4.0 us inter-word gap gives WORD_SLOT_US = 24.0 us per wire word. - Message time: message_time_us = wire_words * WORD_SLOT_US, exact. - Schedule total: sum of the per-message times over the minor-frame schedule. - Utilization: utilization_pct = total_us / frame_us * 100, with the default minor frame FRAME_US_DEFAULT = 5000.0 us (5 ms). - Loading guideline: LOAD_BUDGET_FRACTION = 0.80, budget_us = 0.80 * frame_us; verdict is FITS when total_us <= budget_us, OVER otherwise. - Headroom: headroom_us = budget_us - total_us; headroom_pct = (budget_us - total_us) / frame_us * 100, negative when OVER. - Identity examples: BCRT 16 data words = 18 wire words; RTRT 8 data words = 11 wire words; each wire word costs exactly 24 us. ## Workflow 1. Lay out the minor-frame message schedule as (kind, data_words) pairs, with kind one of BCRT, RTBC, RTRT and data_words in 1..32. 2. Convert each message to wire words with wire_words: the function adds the command and status overhead (2 for BCRT and RTBC, 3 for RTRT). 3. Convert each message to bus time with message_time_us (wire words times the 24 us slot), or read the per-message times from the schedule summary. 4. Sum the schedule against the minor frame with schedule_utilization: it returns total_us, utilization_fraction, utilization_pct, budget_us, headroom_us, headroom_pct and the verdict. Pass frame_us only when the minor frame is not the default 5000 us. 5. Read the headroom percent directly from the summary, or call schedule_headroom for the scalar (0.80 * frame - total) / frame * 100. 6. Confirm the deterministic checks with the contract test scripts/test_mil_std_1553_bus_loading.py. ## Worked example Reference schedule on a 5 ms minor frame (5000 us, budget 4000 us): - BC-to-RT 16 data words: 18 wire words = 432.0 us. - RT-to-BC 32 data words: 34 wire words = 816.0 us. - RT-to-RT 8 data words: 11 wire words = 264.0 us. - BC-to-RT 4 data words: 6 wire words = 144.0 us. Module outputs for this schedule: - total_us = 1656.0 us (432 + 816 + 264 + 144). - utilization_pct = 33.12 percent (1656 / 5000 * 100), verdict FITS. - budget_us = 4000.0 us, headroom_us = 2344.0 us. - headroom_pct = 46.88 percent ((4000 - 1656) / 5000 * 100). - A schedule four times larger totals 6624 us = 132.48 percent, verdict OVER, headroom_pct = -52.48 percent. - Hand check: RT-to-RT 32 data words = 35 wire words = 840.0 us. ## Verification - Confirm wire_words("BCRT", 16) returns 18 and message_time_us("BCRT", 16) returns 432.0 us exactly. - Confirm schedule_utilization on the reference schedule returns total_us 1656.0, utilization_pct 33.12, verdict FITS, headroom_pct 46.88. - Confirm the four-times schedule returns 132.48 percent with verdict OVER and a negative headroom_pct. - Confirm dict keys are exactly total_us, utilization_fraction, utilization_pct, budget_us, headroom_us, headroom_pct, verdict. - Confirm unknown kinds, data_words outside 1..32, frame_us <= 0 and an empty schedule all raise ValueError. - Confirm utilization_pct equals total_us / frame_us * 100 and message time equals wire_words * 24 exactly (identity checks). - Run the contract test offline: python3 scripts/test_mil_std_1553_bus_loading.py (35 tests, deterministic). ## Related leaves - avionics/data-bus/mil-std-1553: the protocol leaf for the 20-bit word encode/decode and format classification side of the same bus. - avionics/data-bus/arinc429-bus-loading: the per-channel loading model for the other avionics data bus standard. - avionics/data-bus/arinc664-afdx: switched network utilization for the Ethernet-based avionics backbone. ## Pitfalls - Counting data words only: the wire-word count must include the command and status overhead (data_words + 2 for BCRT and RTBC, data_words + 3 for RTRT), so a 16-data-word BC-to-RT message costs 18 wire words and 432 us, not 16 words and 384 us. - Charging only the word time: the bus is occupied for the word time plus the inter-word gap, 24 us per wire word; quoting 20 us per word understates a 1656 us schedule by 276 us. - Forgetting the RT-to-RT second command: an RT-to-RT message carries two command words, one more wire word than the BC-to-RT or RT-to-BC forms at equal data words (35 against 34 at 32 data words). - Treating the 80 percent budget as a frame fill target: the guideline leaves 20 percent of the frame as margin, so a 33.12 percent load on a 5 ms frame has 46.88 percent headroom to the 4000 us budget, not 66.88 percent to the frame end. - Judging FITS against 100 percent: a schedule can exceed 100 percent of the frame (132.48 percent in the four-times case) and still be judged only OVER; report the percent utilization and the headroom, not just the verdict. - Reading headroom as microseconds: headroom_pct is expressed as a percentage of the frame length, so it does not equal headroom_us divided by 100 on a 5 ms frame. ## Behavior contract (gate 3) Run the deterministic contract test (stdlib unittest, offline): python3 scripts/test_mil_std_1553_bus_loading.py The test covers the wire-word contract for every message kind at the worked-example data-word counts (BCRT 16 = 18, RTBC 32 = 34, RTRT 8 = 11, BCRT 4 = 6, RTRT 32 = 35), the 1..32 data-word bounds, message times (432, 816, 264, 144 us) within 1e-9, the time identity wire_words * 24, the reference schedule summary (total 1656 us, 33.12 percent, FITS, 46.88 percent headroom, budget 4000 us) within 1e-4, the four-times OVER case (132.48 percent, negative headroom), exact dict keys, determinism, custom frame scaling, the exact-budget FITS boundary, and ValueError rejection of unknown kinds, out-of-range data words, non-positive frames and empty schedules. ## Compliance - Standards referenced, not reproduced: MIL-STD-1553B is the protocol framing standard cited here in reference-only mode; the loading model above is standard bus-scheduling methodology, summary-only per standards-map.yaml. - compliance: STANDARDS-REF, gated: false.