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Thread: Calling all CBM and PET enthusiasts. Need documentation on RS-232 board

  1. #51
    Join Date
    Jan 2012
    Location
    Connecticut, USA
    Posts
    1,486

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    Ok Dave, here are the close up shots of the Crystal and what surrounds it. Let me know if you need additional or better photos:
    http://i613.photobucket.com/albums/t...pslzlbx2f5.jpg
    if you cant tell from the photo the crystal is 10mhz
    http://i613.photobucket.com/albums/t...psnbwhd3u9.jpg

  2. #52
    Join Date
    Jun 2012
    Location
    UK - Worcester
    Posts
    1,975

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    Sorry for the delay. I am somewhat busy at work at the moment.

    Thanks for the photographs. Interestingly, I can't see any specific IC acting as a baudrate generator. This must be undertaken by a divider chain and multiplexer/selector based on standard TTL devices.

    As there is no transmit or receive clock on either of the UARTS, then either there is a fault, or there is something missing that we are not aware of.

    I see from a PM that you have bought an oscilloscope.

    My next move would be to identify where the crystal is connected to - and there also seems to be what looks like a ceramic resonator fairly close to the crystal as well that could be acting as the master clock for the baudrate generator logic (if that is what it is). I would try to identify the logic gates involved in the oscillator(s) and use the oscilloscope to see if they are actually working.

    Don't use the oscilloscope probe directly on the pins of the crystal, it won't damage it - it is just that the capacitance of the probe could change the frequency of oscillation, or stop the oscillator altogether! You need to find the output of a logic gate involved with the oscillator.

    Looking at the logic, it is just possible that the crystal could be used with a couple of TTL inverters to form an oscillator. This frequency is then divided down to produce progressively lower frequencies. The baud rate links will then be used to select one of the frequencies that is then fed to the pins of the UARTS.

    Unfortunately, it is now necessary to get out the data sheets for the IC's, and to draw out the schematic by hand by tracing out the PCB tracks...

    Dave
    Last edited by daver2; August 9th, 2018 at 10:13 PM.

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