disk system hangs on a VAX-750 with SI controller.

[David E. Miran]

We have a problem with our disk system hanging and need help.
We are running a VAX-11/750 with 4.2BSD unix.
The disk system is a Systems Industries 9900 controller with a Fujitsu
eagle disk drive and two CDC 9730-80 disk drives (a sealed 67 M drive).
The controller emulates a massbus device and we use the hp device driver
that came with 4.2BSD.
Occasionally (usually about once a month but last week as much as 5 times
per day) the computer will hang as though a task completion interrupt from
the disk had been lost.  Pressing the reset switch on the controller will
cause the system to pick up running as though nothing had happened.
Another site near us also has seen this problem.  We just installed
a new power supply and the latest firmware in the controller, but it did
not help.  SI suggested using their version of the hp driver, but
(a) did not think it would help and (b) I don't see anything in their changes
that appears to address this problem.
Does anyone know what is going on and what we can do about the problem?
1.  Is it hardware (and if so has anyone identified it well enough for
    SI to fix it)?
2.  Is it software and if so can someone point me to a fix.
3.  I have considered going into the hp driver and modifying it so that
    there is a timeout on pending disk activity (or maybe use the system
    lightning bolt) so that if we wait over one second for a disk interrupt
    we will consider the disk drive to be hung and issue a reset.
    Has anyone tried this and will you send me the code if you have.
    Or when it hangs does the controller become so stuck that a software
    issued reset is ignored and we have to hit the switch.
Any help on this will be appreciated.
-- 
David E. Miran         ...!{seismo,harvard,topaz,ihnp4}!uwvax!uwslh!dem
Wisconsin State Hygiene Lab          (608) 262-0019
University of Wisconsin
465 Henry Mall
Madison, WI  53706