3.1 ARINC 429 Specification, Physical Layer and Topology
Key Takeaways
- ARINC 429 is the Mark 33 Digital Information Transfer System (DITS): a differential, simplex broadcast bus on one twisted shielded pair.
- Exactly one LRU is the source (transmitter) on a given pair; that source may drive a maximum of 20 sink (receiver) LRUs.
- Two-way traffic is not possible on one pair; a return path requires a second, opposite-direction ARINC 429 bus.
- A bus runs at one of two bit rates: 12.5 kbit/s (low speed) or 100 kbit/s (high speed). Speeds are never mixed on the same pair.
- ARINC 429 specifies a 75 Ω ± 5 Ω transmitter output and high-impedance receivers; cable impedance is commonly taught as roughly 60–80 Ω, with no separate far-end shunt terminator.
3.1 ARINC 429 Specification, Physical Layer and Topology
Commission Implementing Regulation (EU) 2023/989 sets topic 5.4, Data buses, at level 2 for B1 and B2/B2L. The current Appendix I does not name individual buses; the pre-12 June 2024 detailed description referred to ARINC, other specifications, and aircraft network/Ethernet. This chapter treats ARINC 429 only. ARINC 629, MIL-STD-1553B, and AFDX are representative study examples developed under that historical scope, not bus names printed in the current consolidated Appendix I. 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). The facts in this section are the ones those items actually probe: what the wire is, who is allowed to talk, how many listeners are allowed, and which two speeds exist.
Mark 33 Digital Information Transfer System
ARINC 429 is the commercial-transport specification for the Mark 33 Digital Information Transfer System (DITS). Aeronautical Radio, Incorporated originally issued it so that line replaceable units (LRUs) from different manufacturers could exchange instrument and system parameters without a custom digital interface for every pair of boxes. In service you meet the same protocol on air-data computers, inertial reference systems, radio altimeters, radio-management panels, flight-management computers, and the display units of an electronic instrument system.
Mark 33 DITS is a broadcast data bus, not a computer network. There is no token, no command/response handshake on the pair, and no multi-master arbitration. One unit speaks; every unit that needs the parameter listens. That design matches cockpit instruments: captain and first officer displays, the transponder altitude encoder, the flight-management computer and a flight-data interface all want the same pressure-altitude word at the same moment, and none of them needs to poll the air-data computer.
Physical medium: twisted shielded pair and differential signalling
The physical layer is one twisted shielded pair. The two signal conductors are named Line A (ARINC +) and Line B (ARINC −). Twisting equalises the loop area presented to magnetic fields from adjacent AC power looms, so induced interference appears as a common-mode voltage on both wires. The braid or foil shield intercepts electric-field coupling and a useful fraction of lightning-induced transients. Shield bonding and grounding follow the aircraft wiring manual and installation drawing. Do not invent a universal one-end or both-ends rule in the hangar — restore the configuration specified for that aircraft.
The transmitter drives the pair differentially. Information lives in the voltage A minus B, not in either wire versus airframe ground:
- A HI (binary 1 in the first half of a bit) places Line A about 5 V above ground and Line B about 5 V below, so A − B is approximately +10 V.
- A LO (binary 0 in the first half of a bit) reverses those polarities, so A − B is approximately −10 V.
- NULL (the return-to-zero rest state) holds both lines near the same potential, so A − B is approximately 0 V.
Typical transmitter amplitudes used in maintenance teaching are HI +10 V ± 1 V, LO −10 V ± 1 V, and NULL 0 V ± 0.5 V (A relative to B). Receivers are given a wider window so a loaded, long loom still decodes: a common teaching set is HI above about +6.5 V, LO below about −6.5 V, and NULL inside about ±2.5 V. Those voltages come from the ARINC 429 electrical specification as used in industry training; they are not numbers that EASA publishes as examination constants. Because the data are differential, a spike that lifts A and B together is rejected — which is why a 429 pair survives in an electromagnetically hostile bay far better than a single-ended TTL or RS-232 link of similar bit rate.
When you put an oscilloscope on the bus, measure differentially between A and B. A single-ended probe from A to ground shows roughly ±5 V and will hide A-to-B shorts, some shield faults, and a missing B conductor.
Source, cable and receiver impedance
The ARINC 429 electrical standard specifies a 75 Ω ± 5 Ω transmitter output impedance, divided equally between Lines A and B. The shielded twisted-pair cable is commonly described in industry material as roughly 60–80 Ω characteristic impedance, often nominally 78 Ω. That cable figure is not an EASA examination constant, and ARINC 429 does not add a separate 78 Ω shunt terminator across A and B at the far end; the source impedance provides the intended matching.
Each sink is a high-impedance load. The ARINC standard requires at least 8 kΩ effective input impedance for an individual receiver and at least 400 Ω for 20 receivers in parallel. Extra receivers and long or poorly repaired stubs can still reduce margin and distort edges, which is why the 20-sink limit and the aircraft wiring manual remain controlling.
Simplex broadcast: one source, up to twenty sinks
ARINC 429 is simplex. On a given pair, exactly one LRU is the source (transmitter). Every other LRU on that pair is a sink (receiver). The source broadcasts each 32-bit word to all sinks at once. Sinks do not transmit on that pair. The specification allows up to 20 receivers on one bus. That limit is a hard examination fact: a twenty-first sink risks pulling the differential amplitude down, slowing edges, and violating the electrical specification.
The geometry is a single-source multi-drop. Wiring diagrams may draw it as a star of stubs from the source connector or as a linear run with taps; electrically it is the same rule. It is not a bidirectional party line and not a half-duplex pair on which anyone may speak after a gap.
Bidirectional data need two buses. If LRU 1 must send parameters to LRU 2 and LRU 2 must send parameters back, the installation uses two shielded pairs: LRU 1 is source on bus X and sink on bus Y; LRU 2 is source on bus Y and sink on bus X. A flight-management computer that consumes air-data labels and emits guidance labels therefore has at least one 429 input and one 429 output, each on its own pair. Two transmitters on one pair produce contention: overlapping HI and LO levels, destroyed parity, and labels that no sink can trust.
The two bit rates
A 429 bus runs at one of two speeds:
- Low speed: 12.5 kbit/s (the specification’s low-speed band is a narrow window around that nominal value).
- High speed: 100 kbit/s.
A given pair is one speed or the other. You never mix a 12.5 kbit/s source and a 100 kbit/s source on the same wires, and you never set an analyser to the wrong speed and then declare the bus dead. High-speed buses use a faster slew rate and are kept electrically short. Low-speed buses use a slower slew rate so longer looms and more stubs can be tolerated without ringing. Bit rate is how many bits leave the source each second; slew rate is how quickly the voltage moves between NULL and HI or LO. They are related design choices, not the same quantity. Common exam distractors are 10 kbit/s, 125 kbit/s and 1 Mbit/s — none of those is an ARINC 429 speed.
LRU roles in the instrument system
Name the boxes the way the specification does. The source is the LRU whose transmitter driver is connected to that pair. The sinks are the LRUs whose receivers tap the pair. An air-data computer is typically the source of air-data labels; the captain’s and first officer’s display units, the flight-management computer, the transponder (altitude reporting) and a recorder interface may all be sinks on that same pair. The same display unit is a sink on the air-data bus and may be a source on a different bus that carries display-control or reversion information.
When you read a loom diagram, find the source LRU first, then list every sink tap and inspect the shield, splices and branches specified for that installation. Maintenance practice follows the same map: a high-impedance analyser may be attached as an extra sink only while the total remains within twenty receivers. Never clip a second transmitter onto a live bus.
429 remains the workhorse of transport-category avionics even on aeroplanes that also carry an AFDX backbone. It is electrically robust, easy to probe with a differential oscilloscope or a dedicated analyser, and the broadcast model matches how instrument parameters are consumed. Level 2 knowledge means you can explain the medium, the simplex rule, the two speeds, the twenty-receiver limit and the source/sink naming — and you can recognise a wiring, loading, or branch defect before you swap an expensive LRU.
Physical-layer teaching summary
| Item | Teaching value | Notes for Module 5 |
|---|---|---|
| Specification name | ARINC 429 / Mark 33 DITS | Broadcast digital information transfer |
| Medium | Twisted shielded pair, Line A and Line B | Differential; not coaxial, not a single-ended TTL pair |
| HI / LO / NULL (source, typical) | +10 V / −10 V / 0 V (A relative to B) | Industry electrical figures, not EASA-published constants |
| Topology | Simplex broadcast | One source LRU, up to 20 sink LRUs |
| Two-way traffic | Two separate buses | One pair cannot carry both directions |
| Bit rates | 12.5 kbit/s or 100 kbit/s | One speed per pair; never mixed |
| Source/cable impedance | Transmitter 75 Ω ± 5 Ω; cable often about 60–80 Ω | High-impedance sinks; no separate ARINC 429 far-end shunt termination |
| LRU names | Source = transmitter; sink = receiver | Identify the source on the diagram first |
On a single ARINC 429 Mark 33 DITS pair, which statement correctly describes the allowed transmitter and receiver topology?
Which pair of bit rates is used on ARINC 429 buses, and how are those speeds assigned to a given pair?
Which description correctly identifies the ARINC 429 physical medium and signalling method used by the Mark 33 DITS?
An air-data computer must send altitude to a display unit, and the display unit must send a barometric-setting word back to the air-data computer. The loom uses ARINC 429. What installation is required, and how should a typical cable impedance figure be treated in Module 5 teaching?