There are articles in STARS about this. The problem is that over the life
of the 3100, there were different techniques involving internal jumpers,
internal dip switches, and external switches.
The same articles may be in DSNlink; I don't know.
Flip the S3 switch on the back of the VAXstation 3100 system -- on the
early models of the first-generation VAXstation 3100 model 30 and 40
systems, this switch was accessable only by opening up the enclosure;
there was no access hole through the rear case of the system -- and plug
the serial line from the VT-series terminal into the printer port MMJ
(DECconnect, DEC423) socket.
Do not use a pencil on the S3 switch as graphite can contaminate the
S3 set `down' sets the graphics console. S3 `up' selects the printer
line as the console.
The various DECconnect adapter part numbers are listed in the OpenVMS
Frequently Asked Questions (FAQ) posting, archived at the following
CompuServe VAXFORUM, Library 0, VMSFAQ.TXT