stray nexus (TR 00) interrupts on Vax 780

[Steve Dempsey]

Here in the UCSD Chem. dept. we have recently brought up
4.2bsd on a VAX 11/780.  With varying frequency we are seeing the
following message on the console:

	nexus stray intr ipl16

Perusal of the kernel source code and the VAX HARDWARE HANDBOOK 1980-81
reveals that the CPU is detecting an SBI interrupt at level 16
from the nexus at TR 00.  This same book also reveals that no nexus
is ever assigned to level 00.  It is reserved for a holding signal
that is guaranteed to be at a higher priority than any other nexus.

We have a Floating Point Systems AP-120b array processor on uba0 (TR 3)
at the indicated level (ipl16, br6 to PDP UNIBUS fans).

Any ideas as to who or what could be generating the phony interrupt?