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Thread: Commodore CBM 2001-16N - BIG MESS!

  1. #51

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    Use pin 20 of the EPROM you have in there, as the trigger for the scope.
    Once you have that in there you should be able to see a nice burst of, full sized, square waves on pins 26 through 33 of the 6502's socket( not on the 6502 pins, on the socket pins of the board ).
    I believe you can use one of the empty EPROM sockets.
    If it looks good in that EPROM socket remove the EPROMs one at a time and place it in those positions, checking the signal again.
    Dwight

  2. #52

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    Quote Originally Posted by Dwight Elvey View Post
    Use pin 20 of the EPROM you have in there, as the trigger for the scope.
    Once you have that in there you should be able to see a nice burst of, full sized, square waves on pins 26 through 33 of the 6502's socket( not on the 6502 pins, on the socket pins of the board ).
    I believe you can use one of the empty EPROM sockets.
    If it looks good in that EPROM socket remove the EPROMs one at a time and place it in those positions, checking the signal again.
    Dwight
    Thank you. This is a very interesting method. So, in this way i am actually checking all ROM Sockets whether the data bus from there to the CPU is working, correct? All ROM Sockets D3 - D9....

    So i did write a 2716 for D8 and a 2732 with your code and without any problem. But its already late and the hobby room currently blocked by my girlfriend (not working on old computers) I will work with the scope tommorow and will let you know.

    Thank you so much for this nice preparation work you did

  3. #53

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    I was going to only do part of it but realized that the instructions my be harder to do than just editing it. I did a bunch of cut and paste to simplify the problem and spent about as much time creating it as I do writing messages. I could have written some code to make the file as it is a trivial pattern but I was not at home where I could do such. There is the pointer to help you understand this format. There are several ASCII formats, of which Intel HEX and Motorola S formats are the most common. It is handy to be able to read each type. I find it is easier to put these in an editor and sprinkle spaces to separate the different fields.
    I did forgot to put in the end marker but most EPROM programmers ignore it anyway.
    Should you want it, it is
    :00000001FF
    It should work well to look at the various buffer or contention issues.
    There are some tricks we can do with the DRAM but we can cross that bridge when and if we get there.
    There are other things we are not dealing with on the simple NOP generator. That is yet another bridge.
    Dwight

  4. #54
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    Dwight,
    your EPROM pattern test is a good idea to test ROM data path. But why not a walking ones test like hex 01, 02, 04, 08, 10, 20, 40, and 80 instead of alternating bits? More shorted data lines could be detected this way. Or were you just making it easier for our OP to create the file?
    -Dave

  5. #55
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    Quote Originally Posted by Dwight Elvey View Post
    I did forgot to put in the end marker but most EPROM programmers ignore it anyway.
    Should you want it, it is
    :00000001FF
    You know your Intel hex format and that the Record Type is 01 for the 'end of file' record. You are definitely an Intel man!

  6. #56

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    Hi,

    just done the next experiment as Dwight suggested. Really hard to get a clear photo to share with you because the scope screen is flickering a lot. Please see the photos attached. Between the spikes there are also a few square wafes but my camera did not catch tem. Basically, on all data lines and across all ROM sockets the scope displayed a similiar picture. I did not get that many of square waves but a lot of spikes. I did get a slightly different picture on D9 but still with many spikes. For me it seems the data bus is in a really bad shape. What do you think?

    scope data bus check 1.jpg
    scope data bus check 2.jpg
    scope data bus check board.jpg

  7. #57

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    I'm not sure what we are looking at. Are you getting a stable trigger on pin 20 of the EPROM. We are only interested in the first part of the signal after the falling edge of the signal on pin 20.
    The signal will be a high speed square wave and only a burst of about 1 millisecond long of a 500KHz square wave. At the sweep rate you are using it will be about 1/2 division long burst. I should have given you the math so you'd know what to look for. The interesting part would only be seen with the sweep at .1 millisecond or faster to see the square wave. You should trigger on the negative edge of the pin 20 pulse by pulling out on the trigger knob.
    We might need to modify the NOP to make the signal more viewable. The burst is quite small and the repetitions are only small amount of the total time.
    Most of these signals is what is happening on the bus that is not being controlled by our EPROM.
    Dwight

  8. #58

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    Quote Originally Posted by dave_m View Post
    Dwight,
    your EPROM pattern test is a good idea to test ROM data path. But why not a walking ones test like hex 01, 02, 04, 08, 10, 20, 40, and 80 instead of alternating bits? More shorted data lines could be detected this way. Or were you just making it easier for our OP to create the file?
    -Dave
    Hi Dave
    This was a working system once. The chance of crossed data line other than adjacent line is quite small. 01010101 and 10101010 should check for adjacent shorts. I'm mainly looking for contention with other devices now. I want to make sure we can do clean reads of the EPROMs. Does this computer use 2716s or 2732s for data. I think it is suppose to use 2732s?
    Dwight

  9. #59

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    Hi Miata
    I don't think you can plug a 2716 or a 2732 directly into a 6332 socket without an adapter socket. Some of the pins will be wrong.
    Dwight

  10. #60

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    Quote Originally Posted by Dwight Elvey View Post
    Hi Miata
    I don't think you can plug a 2716 or a 2732 directly into a 6332 socket without an adapter socket. Some of the pins will be wrong.
    Dwight
    Nope, 2716 should work without an adapter in the 2001N.

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