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