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Thread: PET 3032 garbage screen problem

  1. #81

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    Quote Originally Posted by Dwight Elvey View Post
    Hi Dave
    I'm not sure but the with my understanding of your test, the video Fran poster in #70 would indicate that your test is seeing a bad video RAM. Is that right?
    Dwight
    If you look at the video (#70) you can see how under the garbage are the characters G
    I don't know if this is significant to find the fault

  2. #82
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    But we have ascertained that Fran is not running my test program, so I don't absolutely know what someone else's test program is doing.

    Does the manual for the 'other' Dave's test card shed some light on what the test is actually doing?

    The G's (Good) may be (and probably is) associated with page 0 RAM for instance.


    Dave

  3. #83

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    Quote Originally Posted by daver2 View Post
    But we have ascertained that Fran is not running my test program, so I don't absolutely know what someone else's test program is doing.

    Does the manual for the 'other' Dave's test card shed some light on what the test is actually doing?

    The G's (Good) may be (and probably is) associated with page 0 RAM for instance.
    Dave
    Commodore PET ROM / RAM replacement boards
    This is a board which plugs into the 6502 CPU socket on something like a Commodore PET and can selectively replace the onboard ROM or RAM. I've been making these for a while, and as with all of the boards I make, I keep refining the designs. Adding new features I find I need, or taking requests or suggestions.

    There are three scenarios where one of these boards is useful, as a diagnostic tool, as a ROM/RAM replacement or as a system upgrade. The original use is as a diagnostic tool. ROM and RAM are common causes of failure on vintage computers, and this is an easy way to rule out problems with the original ROM or RAM, buffering, multiplexing or address decoding. If it turns out there are faults with the original ROM or RAM, and replacements are not available, or too expensive, the board can be left in place. In these situations, you can remove all the onboard ROM and RAM and just run with the ROM/RAM board, reducing power consumption and heat generation inside the case.

    The third use comes with older PETs with BASIC 1 or 2, or with less than 32K of RAM. The ROM/RAM board can contain multiple ROM images, so it can be used to upgrade these systems to BASIC 4 and 32K or RAM, or any combination down to BASIC 1 and 8K.

    The idea is you have a system which is not booting, set the board to replace all the ROM and RAM and see if it boots. If not, then you it's back to checking power, clock, reset etc. A lot of the time, that will be enough to get the PET to boot. You can then turn off the RAM and see if it will run with it's own RAM. If not, you need to sort out RAM problems. There are two banks of 16K, and these can be selective controlled, so you can enable the lower bank to get the system to boot and then test the upper bank.

    Once the RAM is sorted, you can look at the ROM. You can select multiple ROM images (up to 16). So if a ROM fault was detected, you needed to identify the culprit by other methods. Now you can enable only one ROM on the board at a time, with the rest replaced, to see which ones work and which ones will not boot. The board can also be used on things such as the VIC20, anything with a 6502 CPU, apart from systems with banked RAM or multiplexed video RAM.

    Supported machines:

    PET 2001-8 (original PET with 9" screen and built in datasette)
    PET 2001N-8/16/32 (with 9" screen and full size keyboard)
    PET/CBM 3008/16/32 4008/16/32 (with 9" screen and full size keyboard)
    CBM 4016/32 8032 8032-SK (with 12" screen and full size keyboard)
    CBM 8096/8096-SK (can be used to test main board, which is an 8032, only if 64K RAM board removed)
    Other PET and 6502 based machines, may need special builds, contact me with any requests

    PET TESTER
    The PET tester ROM on the 2001 and Universal ROM sets can be used to do some simple system testing. It cycles between showing two sc reens.
    Note: if you have a CRTC PET, power up using one of the CRTC BASIC ROM images, then switch to the PET tester and press the reset button on the ROM/RAM board to start the PET tester software.

    The first screen shows a character map
    If any of the graphics are wrong, distorted or flashing, this indicates a fault in the video ROM or supporting circuitry.

    The second is a screen showing the result of testing the first 1K of RAM. G or g indicates a good byte, B or b indicates a RAM fault. If you get any B or b characters, you have a RAM fault in at least the first 1K or RAM (lucky you just bought a RAM replacement board eh, just click on sw
    itches 1 and 2 and you should get a screen full of G or g)

    Fran

  4. #84
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    Ah, so it just shows you the character map - it doesn't test that what it wrote it could read back?

    If this is the case, you are still none the wiser as to whether the CPU can correctly read/write from/to the video RAM or not.

    Dave

  5. #85
    Join Date
    Dec 2005
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    Toronto ON Canada
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    @ Fran: do you (or a friend) have the means to burn Dave's diagnostic ROM into a 2716/2732/2532 EPROM or equivalent?

  6. #86

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    Quote Originally Posted by MikeS View Post
    @ Fran: do you (or a friend) have the means to burn Dave's diagnostic ROM into a 2716/2732/2532 EPROM or equivalent?
    I don't have any eprom programmer but if you tell me what I need I can buy it.

  7. #87

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    I just remembered that I have a ZX-81 for parts with rams working. Is this ram valid for the pet?

    ZX81-RAM.jpg

  8. #88

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    Hi Fran
    We've shown that SA4 gets to the RAMs correctly, yet the display is messed up for SA4. On your original test we could also see a + sign making it back but being inverted. I suspect these are the same locations the g are back from. I think it might be useful to confirm using daver2's test. I believe it also reads the video RAM.
    These test are similar to that I put on my KIM-1 test code.
    Looking at things, these are really great confidence test. They are close to being diagnostic test but lack the ability to make tight loops on the error. This would be more useful for doing tracing. I'm guilty of this flaw as well on my KIM-1 test programs.
    Using a digital scope, we can even capture events before a trigger. I think the test need more trigger points to narrow down where we want to trigger the scope. If we had a logic analyzer it would be easier.
    Dave, think about what I just said. How can we add trigger points to our test? My thinking is that while we are testing one part of the system, we could also use other locations as trigger points. There needs to be two types, one is just a progress point that indicates progress through the test. Others might be error detected.
    As an example, for the video test, we could use reads to the ROM addresses as progress markers or something.
    It needs a little thought but I hope you get the idea.
    It is to make scope triggering easier.
    Dwight

  9. #89
    Join Date
    Feb 2009
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    Southern California, USA
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    2,660

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    Quote Originally Posted by Fran View Post
    I just remembered that I have a ZX-81 for parts with rams working. Is this ram valid for the pet?
    Yes, they look OK.

  10. #90
    Join Date
    May 2011
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    Outer Mongolia
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    Quote Originally Posted by daver2 View Post
    Ah, so it just shows you the character map - it doesn't test that what it wrote it could read back?

    If this is the case, you are still none the wiser as to whether the CPU can correctly read/write from/to the video RAM or not.
    It looks to me like the PET test in that ROM is a variant of the original version I wrote back in 2011, and it indeed does not bother reading back characters from VRAM; you don't *need* working VRAM to get to a BASIC prompt and I wrote this to see if I could get a "heartbeat" out of a PET that was just hard-frozen on garbage. The character display part of the cycle is mainly there to ensure the character generator is producing the expected values/slash/help assess if a data or address bit is stuck. (IE, you can check with the human eye that the character you see at position X is what you'd expect based on the offset from the start of the screen memory; this would also help rule out the possibility of the page zero memory test actually being bad despite the screen filling up with g's because of a stuck address bit is repeating blocks of memory; you'd see repeated chunks of the character set instead of the whole thing four times.) Given that this machine is apparently capable of getting to the BASIC prompt (which would be expected behavior since the "page zero memory test" part of the cycle is all 'g's) the usefulness of that original ROM is at an end.

    Sorry it doesn't do more, it was the first thing I ever wrote in 6502 assembly and I couldn't figure out how to do much more than straight-ahead brain-dead code with neither zero page or a stack.

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