RE: Help...dead 8K PET

From: Chandra Bajpai (
Date: 2001-02-22 03:05:43

Ruud, I agree it must be something with the IRQ.  The machine is an 8K
Chiclet keyboard
2001 PET (2.0 ROMS, 6540 ROM, 6550 it's *real* old).  When I mean
it takes a
long time to come up, the "*** COMMODORE BASIC ***" banner comes up quickly,
but it
sits there for some time (feels like 15-30 sec).  No cursor is visible at
this time.
I suspect if I hit the keys enough it causes the machine to do something and
then the
blinking cursor appears (Is this blinking cusor created in
Software?)...Unfortunately I have
less than a minute to enter your program and execute it (which as you know
with the poor
layout of the chiclet keyboard is hard to do) I am entering the
program, the PET
locks up and displays "rasters" (lines appearing and diappearing accross the
screen, like both the PET & Video circuit are trying to access memory at
once)...the cursor disappears or
turns into a white block.  The only thing I can do is shut off the machine.

The 8K PET uses a 6520 to read the's hooked into the IRQ line
so I guess it generates an interrupt when this happens (I'm really not
familar with the PET at hardware level).
I wonder if something is causing no interrupts to be generated and then
somehow causing too many interrupts to be generated.  Are spare 6520's still

I've always wanted one of these machines, as I got my start in 1979-80 by a
kind gentleman who used to lend his computer to my father so I could play
with it over weekends. Little did he know how much fun a 12 year old could
have staying up and programming for 12 hours straight.


-----Original Message-----
[]On Behalf Of Ruud Baltissen
Sent: Wednesday, February 21, 2001 9:56 AM
Subject: Re: Help...dead 8K PET

Hallo Chandra,

> (1) take a "abnormally" long time for the cursor to come up and start
> blinking after powering on

RESET-circuit ? What happens if you reset the 2001 in the hard way eg by
connecting pin 40 of the 6502 directly to GND. (won't harm if you don't do
it too often)

> (3) Almost within 30 sec the cursor either stops blinking or is solid and
> the computer locks
> up...any ideas on where to look?

IRQ is responsible for the blinking of the cursor AND reading the keyboard.
AT this moment I don't know what IRQ-source the 2001 uses (6532, 6520,
6522), so replace this one at least.
Another check:

10 print"x";
20 for i = 1 to 200 : next : goto 10

If it is the IRQ, then above PRG must keep on running. If it is more
critical, the PRG will stop as well.

Groetjes, Ruud

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