> Does this "ancient" [ 4.0 ] version of ULTRIX support the 
> /2xx DECstations?

ULTRIX 4.0 is PRETTY old.  When our site bought DECstation (model 25s
and 5900s) 5.5 years ago they had ULTRIX 4.3 preinstalled.  We did not
upgraded them with ULTRIX 4.5 before disposal because of it arrived
too late.

> And if yes, how do I tell the machine to boot from the tape?

You have to accustomed yourself with using DECstation PROM command line.
I'm not sure whether you have model 200 (3MAX) or model 240 (3MAX+), then
assuming you have model 240 here;

	>> cnfg 3

You will get system description of system slot (baseboard devices).
When you attached TK50 drive with system chasis, you should get "tz5"
or "tz6" names, where the digit presents the SCSI ID number.  To boot
the system with bootable TK50 media with tz5 device;

	>> boot 3/tz5
 
If you happen to have model 200, the unit has a SCSI adaptor in
different slot, thus should use a different digit as boot argument
like as;

	>> boot 5/(TK50's ID)

There were two ULTRIX, for VAX processor and MIPS processor.  As far
as I can remember now most (not all) of TK50 tapes had "RISC"
disigination on labels.

Your DECstation is a valuable hardware, and NetBSD is the OS for it.
You will find the hardware is useful with it.

Tohru Nishimura
Information Technology Centre
Nara Institute of Science and Technology