hanging DZs

[mike.rice at rand-relay]

From:  Mike.Caplinger <mike.rice at rand-relay>

We have two 750s with a single Massbus and a dual Unibus apiece.  The
only device on each Massbus is a TU78 with dual port kit.  Machine A
has 3 DZs and a UDA50 (with single RA81) on uba0, and nothing on uba1.
Machine B has its 3 DZs on uba1, with the ra81 on uba0.  We run locally
written mods to 4.1 BSD to allow the dual Unibus.  This code appears to
work fine.

BUT, when we access the tape drive from machine B (do a tar, say) some
of the DZs "hang"; that is, you type on the terminal and nothing
happens.  Accessing the tape drive from A works fine.  Resetting the
Unibus with resuba on B doesn't help, and the machine has to be
rebooted to get the machine back.  When I say "some of the DZs", I mean
it doesn't always seem to be consistent.  All of the DZs rarely go out,
but it's never the same one or two.

I don't think it's the dual Unibus code, since we run yet another
Unix machine (pretty much identical with A) dual-ported with a VMS 780,
and have never had a problem with hanging DZs.

So, what's going on?  I'm fully prepared to hear that we've done
something really stupid, but what?  Anyone have any ideas?

				thanks,
				Mike
				(mike.rice at rand-relay)