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

  1. #11
    Join Date
    Dec 2005
    Toronto ON Canada


    Quote Originally Posted by VERAULT View Post
    Kgober, As noted in this other thread:

    The PET DOES NOT have serial support via the user port. thus the reason for buying an external serial device.
    It may not have been built into the OS, but if your software toggles one of the pins on the user port appropriately and can read transitions on another pin appropriately, then you have a serial port. All you really need for RS-232 compatibility is to convert the 0-5 V signals to +- 5 to 12V.

    Many systems of the day "bit-banged" serial ports this way to save the expense of a dedicated UART, baud rate generator etc.; since the CPU had to handle the conversion of data bytes to/from rigidly timed pulses the maximum speed with the relatively slow CPUs of the day was somewhat limited.

    Using a UART relieved the CPU of the details and just accepted/presented finished characters, permitting faster speeds.


  2. #12
    Join Date
    Mar 2017
    New Jersey, USA


    Yes, this is what I was referring to, and the need to bit-bang the VIA is the reason such solutions were limited to lower speeds. But the cost was considerably lower, because all the device had to do was level-convert from TTL to RS-232.

    I had such a device for my C64 although it was considerably easier to use there because the C64 had ROM support for this. On the PET I've heard that such devices came with a ROM that you would have to plug in, presumably to do the same thing the C64 kernal later did natively.

  3. #13
    Join Date
    Jan 2014
    Freedom City
    Blog Entries


    I built my own for the C64 just for the experience building a two layer board at home using Fritzing and the PCB Fab-in-a-box method. Sooner or later I'll fab up a sheet metal enclosure for it.


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