I can't remember if this is true of Airbii or if it's Boeings I'm thinking of, but I remember reading a while back that the three microcontrollers they run on are also from different manufacturers.

No idea about Boeing, but yes for Airbus. Every “computer" is actually two computers, one is the active computer (COM, command) and one is the inactive computer (MON, monitoring). Both still perform the same calculations based on the same input, however they use different software and hardware. There is a watchdog inbetween the two that verifies the results against each other, just in case there is a bug in the hardware or software. Then, you also have multiples of these computers, eg. there are two ELACs and three SECs. The ELACs and SECs are fed data from a different air data inertial reference unit (ADIRU) and use a different hydraulic line to actuate the flight controls. And lastly the results the ELACs and SECs come up with also have to agree with each other or the result is thrown out.

All of that redundency makes it possible to build some really robust flight envelope systems that keep the airplane within safe margins.

* I should note that all of this applies to the A320 family, the systems have been developed even further in recent years. For example with the A350 Airbus made some steps towards allowing the flight computers to be used in Simulators so that the same software/hardware as on the real plane can be used.

I'm a tad late, but I'm curious about where you said

> they use different software and hardware

Does this mean different architectures? If it does my respect for the redundant-hardware approach just went through the window.

Also, how is the watchdog redundant? I can't imagine there's only one; how does this work? Are both watchdogs somehow wired in parallel, are they cross-connected to each other, or...?

It litereally means two completely different architectures, on two physically disconnected computers. Here is a diagram: https://i.imgur.com/Tj0GKbQ.png

Also visible in the diagram is that each side has its own watchdog, both connected to each other. The way this whole thing work is fail safe, so if one computer fails the backup can jump in, and if that fails too the flight controls will either retract or stop in their current position depending on what makes the most sense. It’s also mirrored, so for example if spoiler 2 on the right wing fails and is retracted, spoiler 2 on the left wing will also retract.

Here is a description about the flight controls and how pilot input gets passed through to the control surfaces: http://www.smartcockpit.com/docs/A320-Flight_Controls.pdf

And here is a general overview about the architecture: http://www.skybrary.aero/bookshelf/books/2313.pdf

Oh, wow, that's amazing. Now I understand why avionics are so expensive - verifying the correctness of such a system sounds like a lot of "fun," or at least a lot of time.

(I wonder if there are any systems built on multiple architectures where each unit is itself a redundant system with CPUs in lockstep.....)

Am I to intuit from this diagram that the watchdog watches all the components - power, I/O, memory, and CPU? That's very impressive. Or does it watch a central bus/backplane everything is connected to?

Also, how does either side decide/figure out the other side has failed? Simply deciding that the other half is wrong if it doesn't match this half's output could fail catastrophically if one of the sides reaches this conclusion after entering an invalid state (ie, it's the other side that is correct, and this side is wrong).

I'm also mildly curious as to why the I/O on one side has two connections to the actuators, while the other has only one.