VT-320 interfacing

Joe Piche retrojoe at stateoftheark.ca
Mon Nov 9 18:36:42 CST 2020

Hi All.

I've recently come into a really nice VT-320 terminal, but am having a 
hell of a time getting it to communicate with the outside world (mainly 
my PiDP-8 and PiDP-11). I've gone to the lowest common denominator, 
using a USB serial cable and terminal software to verify everything is 
working before digging into making simh output to a serial terminal.

The VT320 only has the DEC-423 MMJ, no RS-232.

I found a number of guides that show how to wire up the DEC-423 
connector to a DB-9 RS-232, but so far I have struck out. It will 
receive text from the remote computer, but I do not see anything on the 
remote when I type on the VT-320.

I have loop backed the TX to RX on the VT-320 terminal, and it does 
work. TX=RX Baud rate  in the VT320 settings, and the data bits, parity, 
& stop bits match.

DEC-423 (lock tab bottom right, pin one on the left, 6 on the right)
1    DTR
2    TXD
3    TXD-
4    RXD-
5    RXD
6    DSR

For a cross over cable:
423 #2 goes to DB9 #2 (TX to RX)
423 #3 & #4 to DB9 #5 (TXD- and RXD- to GND)
423 #5 goes to DB9 #3 (RX to TX)
423 #1 to DB9 #6 (DTR to DSR)
423 #6 to DB9 #4 (DSR to DTR)

Does this look like it should work? It does leave DB9 RTS and CTS 


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