OT: HP signature analyzer

I've seen HP service manuals that give signatures for various testpoint. They generally have a 'faultfinding' guide of the form 'If the signatature at pin 3 of U6 is not 23H7 then change U1, U8, U12, U24 in orde'. In otehr words they don't tell you exactly waht the fualt is and ovber half the time you'd have guessed it was likely to be one of those ICs (probably including the CPU, ROM and RAM) anyway I have a microprocessor-controlled HPIB extender for whaich the official manual gives signatures for various points. But there's also an undocumented (AFAIK) featuer to disable the CPU data buffer and force NOPs onto the data bus. That way you should get recognisable waveforms on the addres lines, chip sleects. etc. That's how I debug it. Some HP manauls are better htna others. In particular many of the computer nad desktop calculator manuals are pretty pointless. But since I've figured out how to fix some of those without any official technical info, I don't let the fact that they expect me to use a signature analyser to mean I have to use one :-) It's rare, alas, to get firmware listings in service manuals. A counterexample is the manaul for the 4-channel serial card in my P854, the manual for that has a commented source of the 8085 firmware. -tony