>Is it really PHYSICAL not possible to run the DECstation on-board serial
>ports at speeds higher than 384000 bps ?  Or is it more likely a restriction
>in the software, e.g. in the driver ?

It's physically not possible.  The device is a single-chip
implementation of the DZ-11 (which tells the old-timers the whole story).

The DC family chips do DZ-11 compatible speeds, 19600 or some such
ghastliness for EXTA. I think the 5000/200 supplied an external clock
from the System Control Unit, and gets a real, genuine 38400, but it
might be just twice the 19400 or 19600 or whatever-it-is.  (Maybe TLS
knows for sure?)

The decices simply cant go faster than that. Even the 38400 is a
chip-wide mode overlaid on the 19200 speed: toggling a bit on the
system control unit switches the 19200 speed to 38400, so you can't
have ports at both 19200 and  38400 simultaneously: it's either-or.

Simon -- another FAQ entry?
(btw, this is all in the BUG section of the dc manpage.)