4.1 ARINC 629 Digital Autonomous Terminal Access

4.1 ARINC 629 Digital Autonomous Terminal Access

Commission Implementing Regulation (EU) 2023/989 sets topic 5.4, Data buses, at level 2 for B1 and B2/B2L. The phrase “including ARINC and other specifications” comes from the pre-12 June 2024 detailed Appendix I description, which this guide uses as historical study scope. Chapter 3 treated ARINC 429 as the simplex Mark 33 Digital Information Transfer System. This section treats ARINC 629 — the multi-transmitter commercial bus that began as Boeing's Digital Autonomous Terminal Access Communication (DATAC) development and entered airline service as the high-rate backbone of the Boeing 777. OpenExamPrep publishes this material as independent exam preparation; it is not an EASA document and does not claim official approval or partnership.

The live Module 5 paper is a three-option multiple-choice examination (about 75 seconds per question, 75% pass, no negative marking; Category B2/B2L sits 72 questions in 90 minutes, B1 sits 40 in 50 minutes). Examination fees are set by the national aviation authority or the approved Part-147 organisation; EASA does not publish a single official euro fee for Module 5. Level 2 on ARINC 629 means you can explain multi-transmitter topology, autonomous CSMA using terminal and synchronisation gaps, the typical 2 Mbit/s teaching rate, current-mode couplers, and how those choices differ from 429 simplex.

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DATAC and why dedicated 429 pairs were not enough

ARINC 429 is a robust way to broadcast one parameter from one source to many sinks. A large fly-by-wire transport also needs many computers to exchange data with one another: primary flight computers, actuator control electronics, and Airplane Information Management System (AIMS) cabinets. Implementing every path as a dedicated 429 pair produces an explosion of wire, connectors and transmitters. Boeing's DATAC programme specified a bus on which every attached terminal may transmit on the same medium, while still delivering periodic control data at a bounded interval. ARINC published that architecture as ARINC 629.

The Boeing 777 is the heritage commercial installation you should name in Module 5. Later wide-body types moved high-rate backbones toward AFDX, which is covered in section 4.3. 629 did not delete 429 from the 777: many instrument parameters still travel on Mark 33 DITS pairs. 629 is the other ARINC specification on the same aeroplane, solving multi-transmitter, high-rate exchange.

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Multi-transmitter topology

On one 429 pair, exactly one LRU is the source. On one 629 bus, many LRUs are both transmitters and receivers. Each unit that needs to emit a wordstring does so on the shared pair; each unit that needs those data receives them. Typical industry teaching allows up to about 120 terminals on a bus. Treat that figure as a specification/teaching ceiling, not as an EASA-published examination constant.

Installations are dual-redundant. Two independent 629 pairs run through the airframe. A high-integrity terminal transmits the same periodic traffic on both and receives on both. A single cut, coupler failure or shield fault can take one channel; the partner channel continues. That redundancy philosophy appears again on MIL-STD-1553B (Bus A / Bus B) and on AFDX (Network A / Network B), but the access method is unique to 629.

ARINC 629 is not a command/response bus. There is no bus controller granting permission to speak. If you remember only one contrast with section 4.2, remember this: 629 terminals decide autonomously; 1553 remote terminals speak only when commanded.

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Physical layer: the current-mode coupler

ARINC 429 is voltage-mode: a transmitter sets a differential voltage, and sinks tap that voltage in parallel. ARINC 629 is current-mode. The backbone pair carries a signal current. Each LRU attaches through a current-mode coupler (CMC) — a transformer whose bus winding sits in the current loop and whose stub winding faces the LRU.

Transformer coupling is the maintenance-critical feature:

1. The backbone is not broken when an LRU is removed. Unplugging the stub leaves the bus winding in circuit, so the current loop continues. On a series voltage daisy-chain, pulling a box would open the bus; 629 was designed so that is not the case.
2. Galvanic isolation. The transformer separates the LRU electronics from the long airframe pair, improving common-mode rejection and limiting how far a stub-side short can propagate.
3. Multiple transmitters can drive the same loop by impressing current through their couplers, provided the protocol guarantees that only one transmitter is active at a time.

The pair is still twisted and shielded. Twist reduces magnetic loop area; the shield intercepts electric-field coupling and a useful fraction of lightning-induced transients. Shield termination follows the 777 wiring manual. Do not assume a 429 shield-bond rule applies unchanged.

Typical teaching bit rate is 2 Mbit/s. Neighbouring traps are 12.5 kbit/s and 100 kbit/s (ARINC 429), 1 Mbit/s (MIL-STD-1553B), and 10/100 Mbit/s class figures (Ethernet/AFDX). 2 Mbit/s is twenty times 429 high speed, which is why 629 could carry flight-control and cabinet traffic that would have saturated 429.

When you instrument a 629 loom, use the DATAC/629 analyser and breakout fixtures the aircraft maintenance manual calls up. A 429 voltage probe between A and B is the wrong mental model: you would be looking for a voltage-mode signature on a current-mode bus.

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CSMA with a periodic, collision-free cycle

Carrier-sense multiple access (CSMA) means: listen before talking; if the medium is busy, wait. Classic Ethernet CSMA/CD also detects collisions and backs off randomly — which is not acceptable for primary flight-control data. ARINC 629 keeps carrier sense but replaces random backoff with unique timers, so the normal periodic cycle is deterministic.

Each terminal stores three protocol times:

Basic (periodic) protocol

1. The bus is silent long enough for SG to expire. Every terminal recognises a new cycle.
2. The terminal with the shortest TG sees idle for its TG, seizes the medium, and transmits its periodic wordstring.
3. When that transmission ends, idle begins again. The terminal whose TG next expires transmits, and so on, until every terminal that has periodic data in this cycle has spoken.
4. After the last periodic transmitter, silence lasts until SG again — and the cycle repeats.

Because each TG is unique, two well-configured terminals do not decide to transmit at the same instant. That is why Module 5 calls the protocol autonomous rather than command/response. There is no master polling the slaves.

Combined (aperiodic) protocol

Unscheduled messages use the concatenation event. After periodic wordstrings, a CE-qualified terminal may transmit without waiting for a full SG. Periodic traffic still has a bounded cycle; aperiodic traffic fills remaining capacity under the same listen-before-talk rule. If an examination item asks how 629 avoids a bus controller, the answer is unique terminal gaps plus carrier sense, not a token ring and not a 1553 bus controller.

A technician implication follows immediately: a terminal programmed with a duplicate TG can collide with its twin; a terminal whose TG is longer than SG will miss the cycle. Configuration of the DATAC/629 interface is a controlled software/hardware load, not a screwdriver adjustment on the coupler.

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Wordstrings

Industry teaching describes 629 traffic as wordstrings: a label word followed by a run of data words. A typical teaching word length is 20 bits. Do not import the ARINC 429 32-bit field map (octal label, SDI, SSM, bit-32 odd parity) onto 629. Both buses use a label to name a parameter; the encodings differ. Periodic labels on 777 fly-by-wire and AIMS paths refresh every cycle so that control laws see data at a bounded age.

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Maintenance view on the 777

A missing 629 parameter is not automatically a bad computer. Work the medium first: coupler seating, stub continuity, dual-channel comparison, and whether the analyser sees that terminal's wordstring on one bus, both, or neither. If neither bus shows the terminal, suspect the LRU, its power, or a programming/TG load. If only one bus is silent, suspect that channel's coupler, stub or backbone. If many terminals are missing on one bus, suspect the backbone or a coupler/termination fault that has collapsed the current loop.

429 troubleshooting reflexes (count sinks to 20, check HI/LO/NULL with a differential voltage probe, look for a second illegal transmitter) do not transfer. 629 reflexes are: current-mode coupler, unique TG, SG-paced periodic cycle, dual bus, 2 Mbit/s.

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Teaching comparison (contrast only — 429 detail lives in Chapter 3)

If an item asks which ARINC bus allows multiple transmitters on one pair, the answer is 629, not 429. If it asks which bus uses a current-mode coupler and a 2 Mbit/s typical rate, the answer is again 629.