6 Apr
2010
6 Apr
'10
1:39 p.m.
Tony Duell wrote:
The unlikely does occur on occasion.
Only a couple of chips are in sockets in this model/unit, not the problem.
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. My first test would be to bridge the
suspect row/column pin to all the column/row pins (one at a time) with a
bit of wire at the keyboard connector to ensure the problem is on the
logic board.
I seem to rememebr (and it's been years since I've been inside a PET)
that the scanning is doen by an output port on a PIA chip, decoded with a
TTL decoder. And that the inmputs from the keybaord go to the other port
of that PIA/VIA. I would then check that all the outputs of the decoder
are active
As I implied, I already checked all this. As stated in my 1st message and in
more depth in my 2nd message the diagnosis shows a problem with the scan
sequence, not the keyboard.
Anyone have
knowledge of a known failure mode of this sort with these machines?
Many faults on PETs are caused by the very poor IC sockets used.
Replacing them all with turned pin ones will cure said faults, and even
if it doens't help now, (a) it will prevent such faults in th future and
(b) it will tell you that you have a real fault to find.