as I understand it...and I am not an expert but have been exposed to some similar systems just not with Airbus...I believe the following is correct at a systems design level:
* There are three flight 'computers' (boxes) (its more complex than that but that complexity is not germane to your question)
* each box has two entirely different motherboards with different processors and independent software inside of it
* each motherboard takes the same inputs and calculates the appropriate outputs.
* if those outputs disagree, inside of the same box, you get a COM/MON fault and the box/system takes an appropriate action...such as disengaging
* once all of THAT happens in a single box...the boxes are also looking to see if all three boxes are agreeing with each other. This is where you get 'voting.
* if all three boxes agree, great! If two agree, disregard the third. If none agree, execute fault fallbacks.
* If you run out of computers doing things that make sense - shut the computers off and make really loud noises to alert the pilots they are on their own
so...you have the computer agreeing with itself and then you have the computers agreeing with each other. Both are important/critical for fault tolerance.
