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PET 2001 Cursor flashes intermitently

C

crock

Experienced Member
Joined
Jan 12, 2012
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285
Location
Zurich, Switzerland
I have a rather nice condition PET 2001, which mostly works OK. It has the 6550 RAM's and 6540 Basic-1 ROM's.

It boots, shows the correct RAM, and basic programs seem to run OK. However, the cursor flashes intermittently and sometimes stops flashing completely.

That made me suspect the IRQ sources, but with a scope, the V-SYNCH signal from the display looks OK (and anyway the display is stable), and PIA#1 is generating an IRQ every 17ms, which is about 60hz. Also, the keyboard is responding normally and TI$ is incrementing as expected, so I don’t think it’s interrupt related.

Anyone got any smart ideas?
 
crock said:
I have a rather nice condition PET 2001, which mostly works OK. It has the 6550 RAM's and 6540 Basic-1 ROM's.

It boots, shows the correct RAM, and basic programs seem to run OK. However, the cursor flashes intermittently and sometimes stops flashing completely.

That made me suspect the IRQ sources, but with a scope, the V-SYNCH signal from the display looks OK (and anyway the display is stable), and PIA#1 is generating an IRQ every 17ms, which is about 60hz. Also, the keyboard is responding normally and TI$ is incrementing as expected, so I don’t think it’s interrupt related.

Anyone got any smart ideas?
Click to expand...

Really strange. Did you notice if it's position-dependent? I mean, it stops randomly with the cursor in the same screen position or does it stop in certain positions only?
Might be bad video RAM locations?
No better ideas, sorry.

Frank IZ8DWF
 
It sounds like you already went down the road I would expect would reveal the problem.

But if you can check TI$ well enough to know that it's incrementing correctly, you must have enough functionality to watch the cursor blink timer. Unfortunately I can't tell you where it is. It's at $00CD in the '64, but I don't know where it is in the PET.

Am I correct in assuming that everything else seems to be working except the cursor? If so, I'm with Frank; that's just strange.
 
KC9UDX said:
Am I correct in assuming that everything else seems to be working except the cursor? If so, I'm with Frank; that's just strange.
Click to expand...
Well, it seems my initial optimism was misplaced, I think there's more going on here. It does seem to randomly hang from time to time, especially when loading from tape. I've now put my PetVET in and bypassed the ROM and RAM and it's still temperamental, so i'm going to pull all the ROM and RAM on the idea that something's pulling on a data or address line.
 
crock said:
Well, it seems my initial optimism was misplaced, I think there's more going on here. It does seem to randomly hang from time to time, especially when loading from tape. I've now put my PetVET in and bypassed the ROM and RAM and it's still temperamental, so i'm going to pull all the ROM and RAM on the idea that something's pulling on a data or address line.
Click to expand...

Are the PIA 6520 chips on sockets? If so, replace G8 (PIA#1). The lines for cassette #1 and the Vertical Sync interrupt circuit are in that chip.
 
Wow, this is very tricky. Something on the address or data lines would cause more problems than you are seeing. Is it possible that your highly reliable 6502 is a little flacky?
 
dave_m said:
Wow, this is very tricky. Something on the address or data lines would cause more problems than you are seeing. Is it possible that your highly reliable 6502 is a little flacky?
Click to expand...

Depends how hard the flaky device is pulling up or down. I had a flaky 6526 on a 64 some years ago that pulled one of the address lines up just hard enough to be intermittent.

But, I now think it's maybe power related. One of the 5v rails is only a 4.6v, which is below the minimum for TTL. I'll leave it till tomorrow evening now but next stop will be the caps & regulators.
 
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