steve@dartvax.UUCP (Steve Campbell) (08/28/85)
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@dartvax.uucp