PET startup sequence?? (was LF: Commodore PET schematics, troubleshooting info)

From: Dave Dunfield <>
Date: Fri Aug 13 14:56:11 2004

>Hi Dave
> It still sounds like a RAM related issue. The 6502's
>needed to gate the VMA with Phase2 ( as I recall ) to
>get clean address selects. You might put a scope on
>this circuit and make sure it looks like it is working
> You should try to create a more exhaustive RAM test.
>Although, time consuming, GALPAT is just about the
>most intensive.

Hi Dwight,

I've checked all the RAM signals - everything looks OK (about
the same as the other unit).

I tried putting my diagnostic jumper on both the other (working) SP9000
and the PET 4032 that I have here, and both of them exhibit the same
problems with getting the monitor up and running - so I think those
issues are just a red herring and are not realated to the actual fault.

(For some reason, entering 'M 6666' seems to cause a "break" which comes
back with SP=FF and from then on it works OK - on all three machines).

I haven't seen a RAM error yet in my poking around (and I've done a fair
bit), however there could be address bus errors etc. which I would not see.
I really wish I had a better way to load code into the machine then through
the keyboard - I may still do up a RAM test replacement for the kernel ROM.
(Which is a big job, since I don't have a plug-in replacement for the 2332
 so I need to make up an adapter etc.)

Has anyone created a "virtual" tape deck? - The monitor has commands to load
and save memory from/to tape...

I'm still trying to fathom what exactly is happening at power-up - I can't
explain the timing of the "beep" - I thought it might bave been generated
by the interrupt, however with pin4 physically pulled out and tied high (no
IRQ) the beep still happens...

I think the key to understanding what is going on is to figure out exactly
where it's hanging during the initialization of BASIC.

Btw, if I hit 'X' at the monitor prompt, I do get the BASIC 'READY' prompt,
although you can't do anything due to the fact that in this case, BASIC
never got initialized ... Same thing happens on the other machines.

Argh... this is turning into a bigger job than I had expected (It's a lot
easier when things don't work at all) ... I want to get these d*mn things
off my bench - I acquired a 5100 earlier this week which is has been calling
to me ever since I brought it home.

dave04a (at)    Dave Dunfield
dunfield (dot)  Firmware development services & tools:
com             Vintage computing equipment collector.
Received on Fri Aug 13 2004 - 14:56:11 BST

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