[Simh] VAX PSI

[dave porter]

I know almost nothing about VAX PSI, but I have a vague idea
that even though X.25 is asymmetric and intended for DTE to 
DCE communication (i.e., your computer to the phone 
company's premises), it may be possible to coerce it
into talking from one VAX to another (via null modem
cable connecting two devices such as DUP-11s).

I worked on RSX-11M PSI and that's how we tested in
the lab.  I can no longer remember how that was done.
Undocumented option somewhere, no doubt.