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Thread: Altair 88-SIO(C) to Model 33 Teletype

  1. #11
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    Jun 2003
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    Quote Originally Posted by deramp5113 View Post
    Is this a known working Teletype that functions properly today with a different computer?
    Yes, I've had the Teletype working with a KIM-1 a couple of months ago. I'm going to try out the CL->RS232 adapter on the Teletype later so hopefully that will rule that one out.

    Cheers,
    Dave

  2. #12

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    In that case, it sounds like time to drag out a scope and verify the baud rate clock and the shape/timing of transmit and receive data from/to the SIO-C.

    You can also poke around with software to look for possible bus interface problems (i.e., getting some garbage when reading or writing the SIO-C board). For example, single step an SIO echo program like http://deramp.com/downloads/altair/s...er/SIOECHO.PRN.

    Single step the echo program on the front panel until the code is clearly staying in the status poll loop at addresses 0-5. Then press a key on the Teletype and resume single step to see what comes in from the SIO-C board. For the original 8800, look at the data on the LEDs during the final INP cycle of the IN 01 instruction at address 6. For the 8800b, you'll have to look at the data LEDs during the final OUT cycle of the OUT instruction at address 8 (8800b).

    Compare the value received to what you typed. Sometimes a single bit error or two will jump out at you. This could be a bad bus driver on the SIO-C. The 8T97's MITS used frequently degrade or fail. Replace with a 74LS367. Garbage could also be caused by a bad chip in the buffer enable logic for the bus driver.

    If the value received is correct, then the data written to the SIO-C might be corrupted by the receive buffer between the Altair bus and the UART on the SIO-C.

    Finally, of course, garbage could be caused by problems at the serial bit transfer level in which case checking the baud rate clocks and the shape of the waveforms with a scope will probably be required.


    Mike

  3. #13
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    Ok, A little progress I think. Teletype all checks out and I think I've narrowed the problem down to the SIO-C. I've tried the simple echo program. Runs ok, detects a character on the status port but then reading the data port always returns 00h.

    I'm going to check out the address decoding first.

  4. #14
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    Jun 2003
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    I've not found the problem with the SIO-C yet but I decided to configure a SSM IO-4 to act as a SIO board and that seems to work fine. Have managed to get BASIC 1.0 to load from paper tape

    Cheers,
    Dave

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