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Thread: Modifying an SBC 6120 to connect to an ASR 33 teletype

  1. #1

    Default Modifying an SBC 6120 to connect to an ASR 33 teletype

    If you have an SBC 6120 and want to connect it to a ASR 33 teletype, you need to convert the computer to run at 110 baud with 7 data bits, even parity and 2 stop bits.

    The SBC 6120 does not offer configuration options for any of this, but can be modified relatively easily.

    The serial port is driven by a 4.9152MHz 4 pin TTL oscillator. This oscillator is divided down and jumpers select one of four speeds, the slowest of which is 300 baud.

    If you swap the oscillator for a 1.8432MHz part from the same maker, then the 300 baud jumper will give you 111.6 baud speed.

    The serial port is implemented with a Harris 6204 UART and a MAX 232. The UART is capable of supporting the teletype configuration, but the SBC board hardwires it to 8 bit, no parity, 1 stop bit.

    If you carefully bend out pins 35, 36 and 38 of the UART, you can change the configuration by wiring them to the appropriate state. I wired these pins through a slide switch allowing me to switch between 8-n-1 and 7-e-2. Pins 35 and 38 are tied together - if connected to VCC you get 8-n and if you connect them to ground you get 7-e. Pin 36 wired to ground gets you 1 stop bit and tied to VCC provides 2 stop bits.

    This is tested and works great - the RS232 output of the SBC put through an RS232 to current loop box (I used a Telebyte 65) but pretty much any optoisolated converter will work.

  2. #2
    Join Date
    Mar 2004
    Wilmette, IL (north of Chicago)
    Blog Entries


    Hi Carl,

    Good to see you in the DEC forum - thanks for posting this info.
    Last edited by jackrubin; July 17th, 2018 at 02:45 AM.

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