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Thread: Commodore PET Problems, Help to Fix it - Machine Language Monitor on Boot

  1. #41
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    Quote Originally Posted by massaggistefano View Post
    What do you mean by "leave in the kernel ROM (-22) and re-program the 2532 as the $D000 (-21)? You mean that I must leave the "original ROM" and use my one and only working 2532 as a replacement of UD7 (-21)?
    Yes.

    I have a GQ-4X. I must say that I can program almost every EPROM with this one. I am very proud of it!
    Sounds like a good one.

  2. #42

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    Here's what I have if I keep the original KERNEL IC and I replace the -21 (UD7) IC with my 2532 EPROM:

    Screen is no more blank, just a "B*" on the top left.

    And here what my Scope returns at 5 millisecond per division at C4 pin7. (6502)

    https://we.tl/hAuOwIqOkZ


  3. #43
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    Quote Originally Posted by massaggistefano View Post

    And here what my Scope returns at 5 millisecond per division at C4 pin7. (6502)
    Still no Sync. And the start of the Break instruction routine before it bombed. If it was a ROM problem, I would have thought it would be one of those chips.

    Let's make sure the PIAs and VIA are not contributing to the problem. As a quick test, remove both PIAs (C6 & C7) and the VIA (C5) from their sockets. With them removed and if one of them is the problem, we expect to see the Commodore screen. Of course nothing else will be running, no cursor, etc. Be careful not to bend pins.

  4. #44

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    I removed UC6, UC7 (PIAs) and the VIA (UC5)

    The only thing that's changed is that on the screen instead of "B*", now I have on the top left corner "b*" in lower case...

    I am running short of hopes...


  5. #45
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    Quote Originally Posted by massaggistefano View Post
    I removed UC6, UC7 (PIAs) and the VIA (UC5)

    The only thing that's changed is that on the screen instead of "B*", now I have on the top left corner "b*" in lower case...

    Don't give up. There may be a clue there. a 'b' is ASCII 62 hex (0110 0010) while a 'B' is ASCII 42 hex (0100 0010). The difference is in data bit 5. Perhaps there is a problem with D5 in a RAM or ROM.

    Regardless, the next step is to replace the other ROM chips with 2532 EPROMs. After that, we'll see about the RAM chips or making a NOP Generator out of a 40 pin socket. For the moment leave the PIA and VIA chips off until we get the Commodore message on the screen.

  6. #46

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    Okay. Thanks again for your encouragement... I must wait for the arrival of the 2532s. I will let you know as soon as I have them.

    Meanwhile... what is a NOP generator?

  7. #47

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    We need to "sticky" that answer here somewhere.
    Be polite and I may let you live.

  8. #48

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    massaggistefano,
    A NOP Generator is a CPU that has it's Data Lines D[0..7] strapped to the HEX Value of it's NOP Instruction. This causes the CPU
    to continually load the Data (NOP), increment it's Program Counter to the Next Instruction, which steps through all available Address Lines.
    With this easily constructed device, you can test around 85% of the Computers function. The Data Lines won't be tested as they won't be
    connected to the Motherboard from the CPU. But, with a few PULLUP Resistors, you can also check the function of the Data Lines, assuming
    you select the correct direction for the flow of data.

    6502 NOP Generator (untested until PCB is created is at: https://easyeda.com/ldkraemer/6502_N...471f7b8c78479a )
    6502 NOP Instruction is 0xEA

    Shorts on an Address Buss, Data Buss, between them, or from the Power Supply to Ground are another issue.


    Larry

  9. #49

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    Hi,
    I would run the petester.bin image (goes on the kernel ROM socket) and see what happens.
    I recently repaired a 2001-N with various issues (including bad sockets). I made a video with some of the issues and troubleshooting
    process:

    https://www.youtube.com/watch?v=H5_mTETwWJk&t=150s

    I've tried piggybacking on the 4116 but that didn't work, so I decided to run the petester.bin kernel image.
    Also, 4116 can be easily damaged by an intermittent connection on Vbb pin.

    Warning: petester.bin only tests the low 16K (and also not very well, as it didn't identify a bad chip).
    HTH
    Frank IZ8DWF

  10. #50

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    Thank you, Larry. I understand the general idea. If I should use or build this kind of CPU and analyze the results I would need a lot of help!!

    Thanks. Let's wait for the 2532 EPROM, first, and then we'll go on with the tests and see if we should resort to this NOP CPU...


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