POR - Power On Reset.
All the usual disclaimers - I'm not a pilot nor even an aircraft engineer - I'm an MRI/CT Scanner field engineer, but on relevant levels of technology.
It is quite astonishing just how effective the old 'Power On Reset' gag is, with any and all computer driven equipment. Years ago, when computer control was rare or in its infancy, it was possible to know what the computer was doing at any given second - or even millisecond.
Now the computers are so fast and are multiplexing and multitasking at such mind boggling rates, it is not possible for anyone - not even the programmers - to know precisely what it is doing in any given microsecond. If only one of its inputs falls outside the expected range of numbers, the computer can confuse itself to the point where it looses the big picture all together. I imagine this is rarer in aviation products than it is in slightly less critical kit like mine, but computers are computers and they all suffer the same sort of electronic confusion.
Turning them off, leaving them for a short time to let all the capacitors discharge and CCD memories drain away, then turning them back on again can work wonders. Once a computer controlled device has 'glitched up' there really isn't any point in trying to work out exactly what has gone wrong - just POR the sucker. The problem with an aeroplane is; many systems are 'interleaved' and interconnected and it is not possible to Power On Reset individual systems effectively. The communications simply will not restart.
Plus, of course, it is not possible to Power On Reset the aeroplane in the air - only when it is safely on the ground and with hardly anybody or anything inside it.
Incidentally, we used to use a standardised inferface - a PPI, Programable Peripheral Interface - which could lock up so hard that a POR would not clear certain glitches. Only a POR followed by running a specific Diagnostic would shift and clear a particular form of hang up. That was spooky.
Roger.