Unibus zero vectors on vax/780

[Matt Crawford]

We had been getting periodic crashes as the system (BSD 4.2) tried to
do a UBA reset.  The cause was the internal counter of unibus zero
vectors (uba_hd[0].uh_zvcnt) reaching 250000 every 10 days or so.
According to our DECmen, there should only be a few zero vectors per
day or per week.

"Naturally" they suspected the software.  After a few months it was
made clear that the hardware was responsible.  They brought up VMS
for a weekend and hooked up a logic analyzer to verify that the BRRVR
was in fact empty when read.  They have also apparently have tested
a system configured similarly to ours and it behaves the same way.

You may wish to check your system for this behavior.  Our
configuration is: 780 cpu with 1 DW, which holds:
1 UDA50 -- 2 RA81's
3 DZ11's
1 RX211 -- seldom used
1 3COM ethernet interface -- (You may be sure that they tried pulling
				this out.)
The zero vectors come fastest when one or both RA81's are active, and
high activity on the DZ's seems to increase the rate also.  Maybe
this is a design flaw in the UDA50?  VMS users may never observe this
symptom because VMS apparently ignores zero vectors completely.  I
can't tell you for sure what fraction of the unibus or SBI time is
wasted by this problem, but sometimes all terminals halt for several
minutes on end while the CPU and disks remain active.  This could be
related.

You can look at your zero vector count with adb via the command
uba_hd$<ubahd, or by hacking vmshow or a similar program, if you have
it.  I would be interested to know whether other similarly configured
systems have or don't have this problem.
_____________________________________________________
Matt		University	crawford at anl-mcs.arpa
Crawford	of Chicago	ihnp4!oddjob!matt