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Thread: VT320 problems

  1. #11
    Join Date
    Aug 2010
    Silicon Valley USA
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    Quote Originally Posted by lsi11al View Post
    Received a female 6 pin connector today and wired it as complete loop back connector and ran the loopback tests as per the VT320 Pocket Service Guide. Both the communications port and the printer port result in errors. So input to the terminal from the computer works but output to either the computer or the printer port does not. At this point unless someone can point to something I am overlooking I am thinking that there is an internal problem on the control board. Given the directionality symptoms can anyone suggest what chip or chips might be at fault. Also welcome any thoughts on whether this is worthwhile to pursue.

    Thank you.

    Schematics for the VT320 are available on bitsavers in:

    Refer to page 11 of the files, schematic sheet P3. Relevant chips are E7 and E10 9636 dual drivers, and E12 2681 DUART.

    There are separate driver chips, but the serial channels share a common dual UART device. Could try probing the TTL serial outputs of the UART to see if they are OK, and the driver chips are bad. Or possibly the DUART is bad.

  2. #12


    Can you use the terminal with anything else? I know that from a vax 4000 my vt320 needs a null modem adapter or cable.

  3. #13

  4. #14
    Join Date
    Feb 2011
    NorthWest England (East Pondia)
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    Quote Originally Posted by billdeg View Post
    Can you use the terminal with anything else? I know that from a vax 4000 my vt320 needs a null modem adapter or cable.
    He is using an adaptor, and looping 2+3 at the 25-pin plug results in nothing, even though control signals are disabled. I still wonder if the terminal is looking for CTS before sending. Might be worth looping 4+5 as well, and checking that the pin with a voltage on, can't figure out if its 2 or 3 sits at -12v below signal ground (. If its at zero then I suspect that the 9636 used here. However these seem to be available cheaply.

    I know the 75 series RS232 drivers were very prone to popping.

    Looking for Analog Computers, Drum Plotters, and Graphics Terminals

  5. #15


    The latest loopback I did was directly with the 6-pin plug. I looped DSR to DTR, RX+ to TX+, and RX- to TX-. Those are the only signals available from the terminal. This is what resulted in failure of loopback tests on communication and printer ports. When connecting the terminal to the computer the H8575A adapter does loop pin 4 to 5. I guess the next thing to do is open up the terminal and see what versions of the DUART and Dual Driver chips are on board.

  6. #16


    I ordered the driver chips, the DUART, and the 40-pin socket. They finally arrived. Had to completely disassemble the monitor to get at the board. Removed the old chips, placed sockets and plugged in new chips. Reassembled the monitor and with great anticipation turned it on. The good news is it now passes the printer port loopback test. The bad news is that it still fails the DEC-423 port loopback test. Not the result I was hoping for.


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