I now have (at last) found out how the decision where the system console
is on VS/MV 3100's :-)

If the machine has a graphic adapter the prom uses it as console.
If the machine lacks graphic adapter it uses the first serial port as console. 

If a diagnostic switch located at the rear is on the machine will use
the fourth serial port as console port. (Numbered 3 (!) on some MV3100's).

I'm going to put this functionality into NetBSD now ASAP with the exception
that if a color graphics board is found it will use the diagnostic port instead.

-- Ragge