7 Apr
2010
7 Apr
'10
12:05 p.m.
Tony Duell wrote:
The unlikely does occur on occasion.
Of course...
However, assuming the firmware does the obvious thing (load a register
with 0, write it to the output port, increement it, go round again), I am
wondering just what sort of firmware failure could cause this. If the
counter was stored in RAM, it could be a RAM problem, I suppose.
Do you have a logic analyser? If so, try triggering it on a write to the
apporpriate PIA register, and look at machine cycles arround that. See if
you can figure otu what hte frmware is really doing.
Speaking of early Commodore PETs, I have here a
4032 on which some of the keys
do not work. Some time ago I took a brief look at it and, as I recall,
concluded the problem was not the keyboard but rather something was messed up
with the scanning, something like not all columns/rows being scanned. I think
it was a 6821 or 6822 or some such doing the I/O, but the diagnosis was perhaps
more suggestive of something like a PROM bit failure causing the firmware
program to foreshorten the scan sequence.
Possible, I suppose, but unlikely. Only a couple of chips are in sockets in this
model/unit, not the problem.
Far too many times i've thoguht 'Oh, that's certainly not the problem'
and then spent hours/days looking elsewhere only to find in the end that
that it _was_ the problem :-)
I can't rememebr who first said 'In any system that which is most
obviosulty correct, beyond all need of checking, is the problem.'
-tony