what is purpose of MSPARE pins on Q-bus?

[saipan59]

What is the purpose of the MSPARE (Maintenance Spare) pins on the Q-bus?
Pins AK1 and AL1 are connected together on the backplane, as are BK1-BL1, CK1-CL1, and DK1-DL1.
Bus Handbook says they are connected on BP, and not bussed, but doesn't hint at what they are really for.
Only reason I can think of to connect pairs on the BP is for a self-powered module, so that it can detect when it is inserted. But I'm not aware of any module that works that way.

Pete

 

[saipan59]

Um, just found part of the answer:

"LSI-11/2 (M7270) processors use AK1 as a bus timeout enable (MTOE L) and then ground AL1 internally, allowing the logic to function normally. Cutting the connection between these two pins and inserting external logic will allow the bus timeout to be extended."

Now I get it...

Pete