📖 The lesson
Why data buses
Older avionics passed information as analogue signals — synchro voltages, DC levels, AC ratios — over dedicated point-to-point wiring. Every sensor needed a separate cable to every user. As systems multiplied, so did the wiring, its weight and its connectors, which were a major source of intermittent faults. Digital avionics instead send many parameters as coded words along a shared pair of wires: a data bus.
- Serial transmission sends the bits one after another on one pair of wires (the normal method on aircraft); parallel transmission sends all bits at once on many wires (used only over short distances inside equipment).
- Simplex: data flows one way only. Half-duplex: both ways, but only one way at a time. Full-duplex: both ways at the same time.
- Bit rate: bits per second (bps). Protocol: the rules of the bus — word format, timing, who may transmit, error checking.
- A transmitter is also called a source; a receiver a sink. Some buses are controlled by a bus controller; others let terminals take turns.
- Error detection: a parity bit (odd or even number of 1s) detects any single-bit error; a cyclic redundancy check (CRC) detects far more. A receiver discards a bad word and uses the next one.
ARINC 429
ARINC 429 (Mark 33 Digital Information Transfer System) is the most widely used civil aircraft data bus — on the Airbus A320 family, the Boeing 737, 747, 757 and 767 and countless business aircraft, and still alongside newer networks on modern aircraft.
- Simplex and broadcast: each bus has one transmitter and up to 20 receivers. A unit that must reply to another needs a second bus going the other way. Receivers simply pick out the labels they need.
- Cable: a twisted, shielded pair. The signal is differential between line A and line B.
- Speed: high speed 100 kbps for fast or critical data, low speed 12–14.5 kbps (nominally 12.5 kbps) for the rest. One bus runs at one speed.
- Signal: bipolar return-to-zero (BPRZ) with three levels — HI about +10 V for a 1, LO about −10 V for a 0, and NULL 0 V. The return to NULL in every bit makes the signal self-clocking.
- Words are 32 bits long and separated by a gap of at least 4 bit times of NULL.
The ARINC 429 word
| Bits | Field | What it does |
|---|---|---|
| 1–8 | Label | Identifies the parameter — written as three octal digits (for example label 203 = pressure altitude). It is sent first, with its most significant bit first. |
| 9–10 | SDI | Source/destination identifier — which of several identical systems sent the word (for example IRS 1, 2 or 3), or which receiver it is for. May be used as data. |
| 11–29 | Data | The value, in BNR (two's complement binary, bit 29 = sign), BCD (decimal digits), discrete bits or a mix. Unused bits are padded. |
| 30–31 | SSM | Sign/status matrix — the health of the data: normal, no computed data, functional test or failure warning (for BCD words it also carries the sign: plus/north/east or minus/south/west). |
| 32 | Parity | Odd parity: set so that the word holds an odd number of 1s. |
Why the SSM matters
The receiver checks the SSM before using the data. With no computed data (for example an IRS still aligning) or a failure warning, displays remove the value and show a flag, and computers switch to another source. A healthy word with the wrong SSM looks to the crew exactly like a sensor failure.
- Most parameters are re-transmitted continuously at a fixed rate (for example 16–32 times a second for attitude), so a missed word is replaced by the next one.
- ARINC 453 is a faster relative used for weather radar video; ARINC 717 is the format that feeds flight data recorders.
ARINC 629
ARINC 629 was developed by Boeing for the Boeing 777 as a faster, bidirectional successor to ARINC 429.
- Multi-transmitter: up to 120 terminals share one bus and all can transmit and receive — no more one-bus-per-transmitter wiring.
- 2 Mbps on an unshielded twisted pair, terminated at both ends.
- Current-mode couplers clamp around the bus cable and couple to it inductively, so the bus is never cut or spliced to add a terminal. A stub cable runs from the coupler to the LRU's terminal controller. Some installations use voltage-mode coupling.
- Access protocol: there is no bus controller. Each terminal waits for the bus to be quiet for set times — a transmit interval, a synchronisation gap and its own unique terminal gap — so every terminal gets its turn in a fixed order without collisions.
- Words are 20 bits (3 sync bits, 16 data bits and a parity bit), grouped into labelled messages.
MIL-STD-1553B
- The standard military bus since the late 1970s (F-16, F-15 and later aircraft, also helicopters and spacecraft).
- 1 Mbps, half-duplex, Manchester II bi-phase coding on a shielded twisted pair, usually dual-redundant (bus A and bus B). Terminals connect through transformer-coupled stubs.
- Command/response: a bus controller (BC) runs the bus. It sends command words telling a remote terminal (RT) to send or receive data words; the RT answers with a status word. Up to 31 RTs. A bus monitor only listens (for example to record data).
- Words are 20 bits: 3-bit sync, 16 bits of information and an odd parity bit. MIL-STD-1773 is the same protocol on optical fibre (Chapter 7).
Aircraft networks: Ethernet and AFDX
Newer aircraft move far more data — video, maps, software loads, maintenance files — so they use Ethernet (IEEE 802.3) networks built from switches, as in an office, but made deterministic and redundant for avionics.
- AFDX (Avionics Full-Duplex Switched Ethernet, standardised as ARINC 664 part 7) is used on the Airbus A380 and A350; the Boeing 787's Common Data Network is also based on ARINC 664.
- Each LRU is an end system connected by full-duplex links (10 or 100 Mbps) to switches. Because every link is full-duplex and switched, there are no collisions.
- Virtual links (VL): each message flow from one transmitting end system to one or more receivers is a pre-configured virtual link with a bandwidth allocation gap (BAG) — the minimum time between its frames. Switches check every VL against its configuration table and discard non-conforming traffic, so the worst-case delay is known: the network is deterministic.
- Redundancy: two independent networks, A and B. Each frame is sent on both, and the receiver uses the first valid copy.
- Links may be copper (shielded twisted quad cable) or optical fibre.
- Other networks: standard Ethernet with TCP/IP for maintenance laptops, data loading (ARINC 615A), EFB connections, cabin and in-flight entertainment systems; CAN bus (ARINC 825) for smaller distributed systems such as cabin, lighting or some engine and flight-control sub-systems; RS-422/RS-485 serial links inside systems.
- Security: connecting avionics to networks that reach passengers or ground links creates cyber-security risk, so the network is divided into domains separated by gateways and firewalls.
Bus comparison
| ARINC 429 | ARINC 629 | MIL-STD-1553B | AFDX (ARINC 664) | |
|---|---|---|---|---|
| Direction | Simplex, 1 TX : ≤ 20 RX | Bidirectional, multi-transmitter | Half-duplex, command/response | Full-duplex, switched |
| Speed | 12.5 or 100 kbps | 2 Mbps | 1 Mbps | 10 / 100 Mbps |
| Coding | Bipolar return-to-zero | Manchester (doublet) | Manchester II bi-phase | Ethernet frames |
| Word | 32 bits | 20 bits | 20 bits | Frames up to 1 518 bytes |
| Control | None needed | Timing gaps, no controller | Bus controller | Switches + virtual links |
| Example | A320, B737, B747 | B777 | Military aircraft | A380, A350, B787 |
Maintaining data bus wiring
- Keep the twist and the shield intact. Untwist conductors only as far as needed at a termination, and terminate shields exactly as the wiring manual specifies — they are part of the bus's protection against interference (Chapter 11).
- Repair only with approved splices, cable and contacts; bus cables have a controlled impedance, and the wrong repair causes reflections and data errors.
- On ARINC 629 and 1553 buses, terminators and couplers are part of the bus — a missing terminator or damaged coupler can disturb every terminal.
- Faults show up as stale or flagged data, SSM failures, or BITE messages such as "no data from…" — check the source's SSM and label with a bus analyser before replacing receivers.
- Fibre and Ethernet connectors need cleaning and inspection, not just mating (Chapter 7).
B2 extension — multiplexing (5.9)
A data bus is a form of time-division multiplexing: many signals share one channel by taking turns. Inside equipment, a multiplexer selects one of several inputs onto one output according to binary select lines, and a demultiplexer does the reverse. Their logic-diagram details are B2 knowledge.