Re: option ROM into a 3032

On Wed, Sep 27, 2017 at 10:24 AM, Francesco Messineo
<francesco.messineo@gmail.com> wrote:
> Hi all,
> I'm repairing a CBM 3032, I've found all the original ROMs have been
> replaced with EPROMs (but the character generator one). It's probably
> an upgrade to BASIC 4, but I'll find it out after I dump all of them

Hi, Francesco,

That seems likely.

> It has a 2532 EPROM on UD5, so this might be a well known option.

UD5 is part of BASIC 4, at $B000.  BASIC 2 starts at $C000 so you have
3 sockets for option ROMs.

> I attach it a dump of UD5 here

It's just the first ROM in BASIC 4

   http://www.zimmers.net/anonftp/pub/cbm/firmware/computers/pet/basic-4-b000.901465-23.bin

The notes say it's a patched version of the code there.  I don't know
off the top of my head what is different with the 901465-19
"unpatched"/original version.

> in case anyone wants to check what is
> it and upload to the well known archives if it's not yet archived.
> By the way, the fault on this PET is a garbage screen with moving
> patterns, some characters change with cursor blinking and key presses.
> It doesn't seem to react to basic commands though, but I can't think
> of an easy way to check if it's executing commands correctly.

If it changes with keypresses, then it's possibly doing something.
You could plug in a disk drive and try loading a directory.  You could
also try poking values into the User Port and checking the output pins
with a volt meter.

> I think I'll start debugging the video generator circuit (it's
> obviously the non CRTC dynamic board).

Since you have garbage characters, the video timing chain is good, the
chargen ROM is good, and in fact, a lot of the video circuitry is good
or you wouldn't have well-formed characters.  It's not impossible that
there is a fault there and the 6502 isn't successfully writing to
video RAM, but I can't think of a mechanism that part of the screen
RAM works but not all of it.  Dead 2114s are _usually_ just that -
dead.  100% stuck bits.

Do you see any patterns if you hit CLR-HOME?  Every 8th or 16th char
is cleared?  What if you type the same char hundreds of times?  Does
that char appear in regular intervals where it was not before?  Could
be an address decoder or something where the 6502 can't access all of
the RAM at $8000-$83FF

In my experience, when you start up to a garbage screen, the reset
circuit could be affected, or you've got a ROM problem (or a serious
main RAM problem, like the zero page is not working at all).  Check
your voltages.  4116 DRAMs need +12V and -5V to operate.

Oh... if you look up how to ground the diagnostic pin on startup (it's
on the User Port), and the problem you are having is with a bad BASIC
ROM and not a bad KERNEL ROM, you will get dumped into TIM.  I used to
do that when I would be working in Machine Language and I had to reset
and didn't want BASIC to clear my RAM or I'd lose my work.  If the ROM
at $E000 or $F000 is bad, then this trick won't work, but it's easy to
try.

-ethan

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