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Thread: PET 2001 Cursor flashes intermitently

  1. #1
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    Default PET 2001 Cursor flashes intermitently

    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 donít think itís interrupt related.

    Anyone got any smart ideas?

  2. #2

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    Quote Originally Posted by crock View Post
    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?
    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

  3. #3

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    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.

  4. #4
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    Quote Originally Posted by KC9UDX View Post
    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.
    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.

  5. #5
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    Quote Originally Posted by crock View Post
    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.
    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.

  6. #6
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    I swapped the PIA's over and then swapped with known good ones. No difference unfortunately.

  7. #7
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    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?

  8. #8
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    Quote Originally Posted by dave_m View Post
    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?
    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.

  9. #9

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    You may be on to something. Hopefully!

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