🚀 go-pugleaf

We have a similar setup here in CSC's complex in San Diego.  We
are running 3 wire communications, but unlike you we are running
pins 2 (TD), 3 (RD), and 7 (SG) only between the computer and the
terminals.  The RS-232 25 pin connectors have pins 4 and 5 and 
pins 6, 8 and 20 shorted at the terminal end and at the host.

We run our lines several hundred feet in a very noisy environment
without serious problems.  The terminals are operating at 9600 baud.

I believe the problem you are experiencing is due to the transmit line (pin 2)
not being terminated at the data terminal end.  It acts like a very
long antenna.  Any electrical noise, or something like a login
prompt from the host, will be transmitted to the host.  The host,
upon receiving the garbage, will respond with something if a terminating
character is detected.  This response goes out over the receive line (pin 3)
and is picked up by the transmit line (pin 2), etc., etc. etc.

This also can occur when a terminal is powered off and the UART completes
the connection between pins 2 and 3, as well as with an AB switch.  

We solved the problem by having the AB switch short pin 2 to pin 7
on the line to the host that is not connected to the terminal. 
This does no damage to the host port, and keeps it from talking to itself.
------------------------------------------------------------------------------
Ed Embick    (the more paths I make, the more paths they break! waaaaaaa....)
Computer Sciences Corp.                embick@noscvax.UUCP  or
4045 Hancock St.      {akgua,allegra,decvax,ihnp4,ucbvax}!sdcsvax!noscvax!embick
San Diego, CA 92110                MILNET:  EMBICK@NOSC
(619) 225-8401 x516