I am a student and have been researching about ARINC 629 and AFDX. In b777 fly-by-wire system, what kind of cable is used to transmit analog signal from Actuator Control Electronic (ACE) to the respective electrically-controlled actuators? What kind of bus cables/connectors are used in ARINC 629? What about in AFDX? Does AFDX use fiber optics?

  • $\begingroup$ If you specify analog signal it probably won't be fiber. Are you sure it's really analog? $\endgroup$ – user3528438 Sep 14 '17 at 13:53


From The Avionics Handbook, chapter 11, Boeing B-777, Fly-By- Wire Flight Controls:

The ACEs [Actuator Control Electronics] and PFCs [Primary Flight Computer] communicate with each other, as well as with all other systems on the airplane, via triplex, bi-directional ARINC 629 Flight Controls data busses, referred to as L, C, and R. The connection from these electronic units to each of the data busses is via a stub cable and an ARINC 629 coupler. Each coupler may be removed and replaced without disturbing the integrity of the data bus itself.

Triplex, bi-directional ARINC 629 Flight Controls data busses

Triple: The B777 has A629 buses that can be dual, triple (flight controls) or quadruple (engine controllers) for redundancy. A triple bus means the information is send on 3 buses in parallel, using 3 bus interfaces, like this:

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The primary flight controllers have three lanes allocated to different functions: command, monitor and standby. Each lane is connected to a different bus of the triplex.

A629 is in many points similar to thin Ethernet (e.g. 10Base2) but uses a twisted pair instead of coaxial cable for the physical layer:

  • Bi-directional: Contrary to A429, A629 is bi-directional, data can travel in any direction,
  • From any interface to any other interface,
  • The bus is a twisted cable,
  • Terminators are fitted at both sides to prevent signal reflection and stationary waves,
  • A stub connects the unit to the bus using a coupler,
  • Couplers are plugged directly on the cable without cutting the wires.

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Coupler: This is actually a transformer and data are transmitted by induction.

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Data: Data on the bus are digital, coded with Manchester bi-phase.


AFDX (Avionics Full-Duplex Switched Ethernet) is similar to 802.3 Switched Ethernet Standard. The physical layer can be a shielded twisted pair or an optical fiber.

Instead of a bus topology it uses a star topology. Switches are used to connect the different nodes.

enter image description here

The particularities of the switched network:

  • Data don't flow to each units on the same bus, the network can be segmented to reduce contention (collisions).
  • Since the broadcast domain is reduced, this allows for higher data rates.
  • Deterministic transfer times can be achieved, something which is important in real time applications.

AFDX also allows for virtual links, that is defining a point to point virtual circuit, to completely abstract the network layer.

  • 2
    $\begingroup$ Good answer but you didn't answer the "what kind of cable is used to transmit analog signal from ACE to the respective electrically-controlled actuators?" The analog signal cables depend on the signal type. Discrete signal I/O is typically a single wire. Voltage differential signals (proportional control) is typically shielded or unshielded twisted pair. UTP is preferred to reduce weight, but STP may be required depending on frequency response and potential interference. $\endgroup$ – Gerry Sep 15 '17 at 17:27
  • $\begingroup$ @Gerry: ah yes indeed, I missed completely the part between ACE and PCU (I had this page bookmarked, likely because I wanted to update it, but see the bookmark only today!). The analog connection is multiwire, here is the connector on one of the rudder actuator. In addition of the position, there are wires to select the actuator mode, and for feedback (power metering and cancellation of actuators fighting against each other). $\endgroup$ – mins Jan 17 at 17:35

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