Updates on repairing Xerox Alto
ard at p850ug1.demon.co.uk
Thu Oct 21 13:53:51 CDT 2010
> Thanks everyone, I guess I have no choice but to attempt to not only get
> all the cards in the alto to work, but also get the disk drive and disk
> pack working properly... Lets hope just a couple components are bad,
> that are easily diagnosable...
Provided the PSU is good (and you have checked that, right :-)), I can't
see that plugging all the boards in can do any damage. OK, when it
doesn't work, you don't know immeidately where the problem is, but it
shouldn't be all that hard to identify sections that are working.
Somebody mentioned the trick of tracing the microcode with a logic
analyser. This is something I've done many times (not on the Alto, since
I don;t have one, but certainly on PERQs and HP9800s). Assuming you have
a sorce listing of the microcode, this will tell you a lot about what
it's doing or not doing.
I understand that the Alto doesn't alas, have a test connector carrying
the microcode address bus (the 2 machines I mentioned both do). So you
have to pick the signals off the backplane
If you don't ahve a logic analyser, then another trick that can help is to
connect the microcode address lines to one set of inputs of an <n> bit
comparator. The other set of inputs comes from a suitable set of DIP or
thumbwheel switcehs. Feed the 'equals' output of the comparator to a
logic rpboe or a monostabel and LED.
The idea is that while you can't trace the microcode, you can at least
see if certain routines are being executed (set the address of the start
of the rotine on the switches, if the logic brobe is triggered, then the
microde is gettign to that address). Does it ever run any of the video
routines, things like that.
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