VAX 750 autoconfig hang

[Steve Campbell]

There's an annoying problem with our VAX 750 running 4.3BSD.  It has a
MASSBUS (RH750) and a TM78 tape handler attached to it.  Here's the config:

controller	mba0	at nexus ?
master		mt0	at mba0 drive ?
tape		mu0	at mt0 slave ?

All's well until we have to disconnect the TM78 from the 750 for a while.
We unplug the monster MASSBUS cable from the back of the 750 CPU and put on
a dummy connector.  The next time we try to reboot the 750, the autoconfig
code gets as far as printing "mba0 at tr4" and then hangs looping.  If we
press the 750's RESET button, the reboot succeeds, printing the same
message about mba0 but then happily going on to other controllers.

It would appear that something in the autoconfig code is waiting for something
that never happens (an interrupt from the TM78?).  Halting the processor
on one occasion showed the CPU to be in routine mbintr in vaxmba/mba.c.
By the way, the problem occurs in two 750's but not the 785's.

Any suggestions or explanation would be appreciated.

							Steve Campbell