[ranger207]

> The self-checking pairs ensure that if a CPU performs an erroneous calculation due to a radiation event, the error is detected immediately and the system responds.

How does a pair determine which of the pair did the calculation correctly?

[AlotOfReading]

It doesn't have to. It raises an error that the system can detect and take action on. Usually that'll be some combination of interrupt/reset and an external pin to let the rest of the system know what's happened.

[ranger207]

What raises the error, and how does the system know that an error has happened? Like, if you have two processors calculating 2+2, and one comes out to 4 and the other to 5, how does the system know which one is correct? Actually, typing it out I think I get it now. It doesn't need to know which one is correct, it just has to redo the calculation if there's ever a disagreement. Then if somehow both processors calculate 2+2=5 simultaneously, the next computer over will disagree and everyone will repeat the calculation, and that's why they have 3 levels of paired redundancy and the chance of 8 simultaneous single-event upsets is low enough for their risk tolerance. Ok, now I get it.

[Ductapemaster]

In simple terms, this works by doing an XOR on the outputs and if they disagree, performing a fault recovery.

There's also space systems that use 3 processors and a majority vote for the correct output, but that's different.

[SauntSolaire]

You just run the calculation again until both agree.