Trouble with 2 UBAs & 2 UDAs solved

[Steve Campbell]

In June I posted a request for help with instability we had experienced
on our 750 under 4.2 when we installed a second Unibus, second UDA50,
and four more RA81s.  Symptoms included corrupted disk data and crashes,
especially during disk/tape operations between the second set of disks
and the TU78 (Massbus) tape unit.

Thanks to all those who offered help.  There was some good advice, but
nothing solved the problem.  We converted the machine from 4.2BSD to
Ultrix 1.0 and then Ultrix 1.1 in hopes of finding a fix (and for other
reasons), but to no avail.

Today our DEC FE was rechecking the machine and found the source of the
problem.  Under our original configuration - 1 UBA and 1 MBA - the
MBA was set (correctly) at CMI [CPU backplane] arbitration level 3.  Now
the board that is the second UBA, the DW750, has a fixed arbitration 
level of - you guessed it - 3, so when it was installed, the MBA's level
should have been changed to something else.  It wasn't, and the two
i/o busses were at the same level.  The result was chaos, especially
when those 2 busses were involved in a high speed copy operation, like
dump(8).  The FE changed the MBA's arbitration level by changing a jumper,
and the problem disappeared.

The lesson we've learned is always to recheck all strappable options
whenever hardware is changed, especially when the system configuration
changes.  Obvious, right?

					Steve Campbell
					Dartmouth College
					steve at dartvax.uucp