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Thread: Unique S-100 computer (Z-80/Vector Graphic/Micropolis)

  1. #21

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    From what I can see looking at the jumpers on your PROM/RAM card and the CPU board, the system is configured as a very standard VG system. During the first three bus cycles after power on or reset, the PROM/RAM board responds with the first three bytes starting at E000h (in the monitor PROM). These first three bytes are a jump to the initialization routine in the monitor PROM. The init routine initializes the video display and then puts up the command prompt. The monitor only requires the 1K RAM that is on the PROM/RAM board for operation.

    The PROM/RAM board can’t force a jump to the boot PROM on the Micropolis FDC (at F800h)

    Now that you’ve checked the power supplies on each board, let’s look at other simple possibilities.

    1) The CPU is not reset/running properly. An ECO in the manual for the CPU board suggests putting the axial 22uf cap back in the reset circuit (just to the left of the CPU - you can see it has been removed on your board).

    2) I assume you’ve tried pushing the reset button on the back as well as simply cycling power?

    3) The PROM/RAM board or the EPROMs or RAM chips on the board have a problem. Carefully remove and re-insert the EPROMs and the two RAM chips a few times to freshen up the contact between the pins and sockets.

    If none of these tests reveal anything, do you have an oscilloscope?

    Mike
    Last edited by deramp5113; October 26th, 2019 at 10:00 AM.

  2. #22

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    I have completed the following:

    • Cleaned all board edge connectors and sockets
    • Found a radial 22uf cap and put it back in the CPU board at the spot you mentioned
    • Reset button on the back does nothing at all.
    • Re-seated every IC on all 3 boards. The Z80 was really stuck in there good, tried to be extremely careful but when I worked it out, one of the legs was broken and stuck in the socket. I managed to run a wire from that CPU pin to the socket pin on the backside of the board. Continuity test passed.

    None of this seemed to help. Recycling the power presents the same garbled text screen. The text is basically the same every time I boot.

    Unfortunately, I do not have an oscilloscope. I can borrow one from a friend, its an analog scope. What should I check? I may need his help.
    Last edited by nullvalue; October 26th, 2019 at 12:11 PM.

  3. #23
    Join Date
    Feb 2017
    Location
    Zürich, Switzerland
    Posts
    173

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    Hey, I was just talking to someone on Reddit with one of these! (/jk)

    I can't offer any additional advice but I can strongly recommend the cheap Chinese logic analysers like these: https://www.banggood.com/USB-Logic-A...r_warehouse=CN

    They're only eight channels, which means you have to choose between watching the data bus or watching the low eight bits of the address bus, but they work out of the box with PulseView and take so much guesswork out of debugging this stuff. Plus, you can't really go wrong with $8 delivered. They're fantastically useful for all sorts of other jobs, like logging and analysing serial connections, too.

    A logic probe, too, is amazingly useful for showing you whether a line is low, high, or has traffic: https://www.banggood.com/Digital-Log...r_warehouse=CN

  4. #24

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    Thanks again for your help over there! I may need to invest (very little) Iin these. Though my friend did offer me to keep the scope hes bringing over. Do these things do anything that a scope doesn't?

  5. #25
    Join Date
    Feb 2017
    Location
    Zürich, Switzerland
    Posts
    173

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    They record digital signals on multiple channels. The one I linked to streams data to a PC via USB, so it'll record an unlimited amount of data. Connect it to the data bus and you get a complete record of every single instruction and value passing across the bus during the execution of a program. You can then examine the trace, attach decoders for various protocols like UARTs, SPI, I2C, etc after the fact. It makes debugging, e.g., boot sequences very easy.

    Ideally you want to record every signal on the bus, but with 16 address lines and 8 data lines plus a handful of signalling lines you end up needing a lot of channels. I've seen 16-channel ones for about $100 and that's still low end. Those little 8 channel ones are very cheap, very easy, and really useful and I wish I'd gotten one years ago.

  6. #26

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    Quote Originally Posted by deramp5113 View Post
    If none of these tests reveal anything, do you have an oscilloscope?
    I have the oscilloscope now, could you give me a few pointers on what I should try checking first?

  7. #27

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    First, make sure the Z80 reset line (pin 26) goes low and then high when you press and then release the reset button. It looks like your board has been strapped to use the standard PRESET line from the bus (pin 75) instead of the "special" short reset from the DRAM card (pin 55). This means you should be able to see the RESET pin on the Z80 go low when you press the reset button on the back.

    Second, verify the Z80 is getting the 4mhz clock on pin 6.

    Mike

  8. #28
    Join Date
    Jan 2010
    Location
    Central VA
    Posts
    4,706

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    Get a quality logic probe before you bother with a logic analyzer. I've never personally owned one of the cheap Chinese analyzers, but I've helped people troubleshoot systems that were using them, and in two different cases, the cheap Chinese logic analyzer was causing additional problems with the system. 90% of old computer problems can be solved with a logic probe and multimeter anyway. I've got a big 80-channel HP logic analyzer, and it's one of those things that sometimes you absolutely have to have, but mostly it doesn't live on the bench.

  9. #29
    Join Date
    Oct 2016
    Location
    Dutchess County, New York, USA
    Posts
    46

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    Since this is a C-M industrial computer the ROM's might be custom programmed and not for booting to a prompt. Try finding a download of the Vector Graphic standard boot ROM's. If you find the download, I'm sure one of us would help with doing the burning or pick up a TL866 programmer to program the ROM's using a PC. A copy of the CPU manual can be found at: http://s100computers.com/Hardware%20...Vector-Z80.pdf

  10. #30
    Join Date
    Feb 2017
    Location
    Zürich, Switzerland
    Posts
    173

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    Is there a decent S100 emulator, allowing virtual cards to be installed, configured, etc?

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