PET 4032 keyboard scanning problem / was Re:Anyone have an image of a Japanese PET chrgen ROM 901447-12?

[Brent Hilpert]

M H Stein wrote:
> 
> -----------------Original Message:
> 
> Date: Mon, 05 Apr 2010 16:28:58 -0700
> From: Brent Hilpert <hilpert at cs.ubc.ca>
> Subject: PET 4032 keyboard scanning problem / was Re: Anyone have an
>         image of        a Japanese PET chrgen ROM 901447-12?
> 
> 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.
> 
> Anyone have knowledge of a known failure mode of this sort with these machines?
> 
> --------------------Reply:
> 
> The most common problem is just oxidation and/or wear of the rubber 'contacts'
> and the PCB grid; if you're lucky a good cleaning will clear it up, in case of bad
> wear of the conductive pads you may have to recoat them with conductive paint
> ot glue little pads of aluminum foil or aluminized mylar on to them.
> 
> Electronically they are a normal scanned matrix, driven by a 6520 PIA (UB12?)
> and a muliplexer; if there is a problem there then it would most likely be with all
> the keys in a certain row or column. I suppose it's possible, but I'd be very
> surprised if it were a bad ROM.
> 
> Check the schematic and you should be able to jumper across the appropriate
> keyboard lines to test all the rows and columns.
> 
> http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/8032/8032029-03.gif

I had already checked the keyboard, it wasn't as simple as that.

Your mention of the multiplexor reminds me (I had forgotten and thought the
6520 lines drove the scan directly) that the binary scan count from the 6520 to
the multiplexor didn't seem to count high enough to do the full scan, more
suggestive of a program error.

It was a couple of years ago, and I didn't get through a full diagnosis.
I was asking in case there was a known problem with the PROMs failing and
producing random symptoms.

... pulled out my notes from 2004: I had scoped the lines from the 6520 to the
'145 1-of-10 decoder, the observed 4-bit scan sequence driving the 145 is
09ABCDEF0 rather than 0->9, so only 2 lines instead of 10 on one side of the
keyboard matrix are being scanned.