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Thread: Custom 4K ROM for Big Tandy computers like the Model II

  1. #1
    Join Date
    Mar 2017
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    Eindhoven, The Netherlands
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    Default Custom 4K ROM for Big Tandy computers like the Model II

    Hi,

    As I am working on the 512K RAM board for the model II I also needed a way to test it.

    For that reason I did build a 4K ROM board that connects to the original ROM socket using a flat cable.
    Working on the custom ROM made the RAM board development come to a temporary halt.
    The ROM board has room for 2 ROMs (2716 or 2732). So the original ROM can be used without too much trouble. Just change a jumper and reset the system.
    I used a ZIF socket for the second ROM, making inserting a new EPROM very easy.
    By bringing the already present address select for the 2nd 2K to the board, I am able to use a 4K chip. I showed to board to some people at TA18.
    By now, BigTMon as the latest content for my ROM is called, includes RAM bank test, keyboard scan code test, RAM display and edit, full port scan and serial port communication.
    It will also display a message when the Model II has no RAM memory available.
    Using the serial port to hook up another Tandy computer (Model 102), I can run the monitor program without a Model II keyboard connected. Useful when some keys on your keyboard no longer work.
    Also the Model II can now be used as a basic serial terminal by using the new ROM.
    Attached is a picture of the board hanging on the back of my Model II and a picture showing the Model II, Model 102 combination (with the keyboard DIN connector not connected)
    The board: Attachment 53394
    Screen shot: Attachment 53395.
    The code for BigTMon is written in Z80 assembler and assembled using ZMAC. Testing is performed using the TRS80GP emulator and on my Model II.
    The screen shot shows that my machine is currently equipped with 2x 64K RAM board and a HD controller (present in bank 14 and 15). A HD controller has incomplete bank decoding, hence it shows up twice.
    Until now the new ROM board has only been installed in a Model II, not yet in a 12, 16 or 6000.
    One of the things I learned using the monitor was that you cannot map memory bank 0 to the upper 32K of RAM. When you try, there will be no RAM active in that location.
    You can only map banks 1 to 15 in the upper 32K of address space.
    When carefully reading the technical manual of the model 16 it becomes clear why this is the case. By mapping bank 0 to the upper 32K, this address space becomes available as the window to the memory for the 68000.
    I had not yet taken that into account in my first 512K board design.
    One of the tests I'm considering to incorporate is testing for presence of a working video card.
    This leads to the following question; will the monitor get damaged when the Model II is powered up without a video card (or with a defective card), resulting in no video signal being generated?
    Does anybody have experience with that?

    Thanks,
    LaserVision

  2. #2

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    LaserVision, it appears that the image attachments are broken.

    This is an awesome project. Am I to understand that you have full serial terminal support now built into the ROM so we out run a headless Model II/16?

    I look forward to seeing this in action at Tandy Assembly 2019!

  3. #3
    Join Date
    Jun 2010
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    Vancouver, BC, Canada
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    Default

    Quote Originally Posted by LaserVision View Post
    One of the tests I'm considering to incorporate is testing for presence of a working video card.
    This leads to the following question; will the monitor get damaged when the Model II is powered up without a video card (or with a defective card), resulting in no video signal being generated?
    Does anybody have experience with that?
    I looked at the Model II technical bulletins here: https://github.com/pski/model2archiv...Searchable.pdf

    II:025 suggests damage will occur. At least in the case of a VDG board (the one with the 6845 on it) that is not initialized by the Z-80.

    An optimist might suppose that with no VDG board installed there may not be a problem. That would be a sensible design and one might guess that a video board not being driven will just do nothing.

    On the other hand, I can't remember where I saw it (the technical reference manual, maybe), but there was some advice to the technician to be careful when working on the machine. If upon power-up you hear the CRT whine heavily you have about 3 seconds to shut it off before damage occurs. That may have been in the context of repairs specific to the VDG board or just general advice; I don't quite recall.

    Maybe other service bulletins or repair guides imply what happens without a VDG board.

  4. #4

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    To be safe, I suggest unplugging the CRT analog board card edge connector if you plan to run without a VDG as the CRT does have a minimum sync requirements. A defective VDG could certainly damage the CRT if the sync signals are out of spec.

  5. #5
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  6. #6
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    Pete,
    Unplugging the CRT analog board is an excellent suggestion.
    I would indeed feel much better with the CRT without power when performing tests.

    LaserVision

  7. #7

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    Quote Originally Posted by LaserVision View Post
    Pete,
    Unplugging the CRT analog board is an excellent suggestion.
    I would indeed feel much better with the CRT without power when performing tests.

    LaserVision
    What can I do to get one of these boards?

  8. #8
    Join Date
    Jun 2010
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    Vancouver, BC, Canada
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    Quote Originally Posted by pski View Post
    What can I do to get one of these boards?
    Whatever it is, I'll do it twice.

  9. #9
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    Hi guys,

    Good to hear there is some interest.
    Would you opt for a bare board plus programmed GAL (for decoding) and assemble the board yourself based on the provided schematic?
    Or would you prefer a completely assembled and tested board, including BigTMon in EPROM, flat cable (at desired length) and ZIF socket?

    Of course I'll bring complete boards to TA19, but that is still some months to go /

  10. #10

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    I’d prefer the completely assembled option. I have enough projects sitting around yet to be built!

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