Updates on repairing Xerox Alto

[Tony Duell]

> 
> 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.

-tony