Does Ultrix have one device, but has a non-exclusive mode where something can
be ``locked out'' when something else wants to use it?

getty opens /dev/tty00.  Set non-exclusive mode.

cu opens dev/tty00, without setting non-exclusive mode.  The kernel ignores
getty's connection now, and cu owns the port, until done.  Then getty
gets it back.

I could be way wrong here, but...  ;)