Your OS depends on the keyboard. HP 400s were the "hybrid" models made
after HP acquired Apollo, so from the factory they were set up to run
either HP-UX (9 or earlier) with a HP-HIL keyboard or mouse, or
DOMAIN/os (Aegis) with a Domain keyboard. Switching between the two
requires keyboard/mouse swap and setting one value in the NVRAM.
I've only used DOMAIN on my 425, since HP-UX of the early '90s has been
described as not spectacular. SR10.4 works well, and with the y1k997
patches it seems to tick along just fine in the 21st century. DOMAIN is
neat since it's so network-transparent and very unusual, but with the
BSD or SysV environments the learning curve is not nearly as big as
with the Aegis environment.
Of course, NetBSD can be run on them with a HP-HIL keyboard setup as
well, but I think it's pretty evident which camp I'm in. Bear has some
manuals on his site (
www.typewritten.org)
Scott