Svarin:
What's taken place inside FCPC's and FCSC's is not open to the public,
The only thing we know is: What is going in and what should be coming out.
I know the system only like it is advertised to operate in normal and outside the normal conditions.
If system had act like you say it did, it is NOT how it is advertised and would be a serious flaw.
There's only 1 PRIM in control and that will be the one which is capable of computing the highest level of law. (=NORMAL LAW, ALTERNATE, DIRECT).
The priority order is PRIM 1, 2, 3, FCSC.
This would maybe be the easiest part to design, and is common in all kinds of systems having multiple controllers.
Just take the output of controller 1 and use it as inhibiting input for the other controller(s). There you would have ensured the only 1 in control logic.
This inhibit could fail so it would be monitored and a message would be set if it did.
There is only 1 in control doesn't mean the others are doing nothing, the others are computing the 'same' output but this output is not used for servo command but for monitoring the output of the PRIM in control.
These other PRIMS are in MONITORING mode.
If the MONITORING PRIM's decide that the output of PRIM in CONTROL is not in agree with their own output a message would set and PRIM in CONTROL role will be transferred to the next PRIM acc. the priority rules.
At that time there were no messages present which would justify a PRIM in CONTROL change.
The wiring issue of FCPC 2 would possible be a reason to outvote this PRIM for taking CONTROL until there was no other PRIM left.
PJ2:
Any "partial input/control" by other than the Master FCPC is prevented "by design".
Svarin:
Not quite. Especially on elevator control, the need to activate all servos simultaneously under certain conditions make it necessary to cater for dual PRIM outputs onto parallel servos. Such thing is therefore not positively excluded from the design.
You are right by saying that (elevator only) all servos can operate simultaneously under certain conditions e.g. when inb. servos are unable to perform the commanded elevator position (due to aerodynamic load) the outb. servo’s (controlled by the executing part of PRIM 2) will assist and become active (i.s.o. dampening) parallel to Inb. (PRIM 1) servo's.
But NOT in the way you suggest (2 PRIM's) in control at the same time.
To explain this behavior you need to know a PRIM consist of 2 seggrated parts, a CONTROL part and an EXECUTING part.
For the CONTROL part there remain only 1 control and it is this 1 in control which demands the EXECUTING part of the other to assist its own EXECUTING part.
There are more situations were PRIM 1 in CONTROL needs output confirmation of other PRIM (e.g. Ground Spoiler) or other specific PRIM 1 function but that would take place in another flight phase.
Image below could clear some mis-interpretations,
Crosslinks between PRIMS and FCSC inputs are omitted for clarity.
