Omrom programmable terminal

Self-diagnostics always worry me in that you're using a faulty device to tell you what's wrong with itself. How do you know the bits needed for the diagnostics are working correctly? To be fair well-designed self-diagnostics start by assuming that only a tiny part of the machine is working (and testing that), and then going to using more and more features, but never depending on a feature that's not been previously tested. But an awful lot of self-diagnositcs are not well-desiogned. I've repaired at least one HP instrument where, if one of the RAMs had failed in a particular way, it did not give the expected 'RAM U<n> failed' pattern on the LEDs, but something totally different. Fortunately I only got a list of the erorr codes _after_ I'd found the fault by the old-fashioned method. Some of the HP9000/200 power-on self test error messages are little weird. I got confused by erorrs relating to the disk controller in 9836, for example. If I was a board-swapper, I proably would have replaced the rgith FRU first (namely the FDC board), but the message pointed in totally the wrong direction for component-level repair. -tony