pdp11/84 PMI memory: What is the problem with Q bus?

Chris Zach cz at alembic.crystel.com
Wed Apr 22 19:09:37 CDT 2020

> So whatever the fault is (perhaps the QBUS block transfer issue reported
> up-thread), it must _seem_ to work, but fail in actuality. It would be
> interesting to appply a logic analyzer, and see what the bus transaction
> looks like, if it looks OK on the bus (in which case it's an internal
> failure).

One possibility is that it's not handling multiple DMA devices talking 
at the same time or calling to arbitrate the bus. Right now I have an 
RL02 controller and the MTI controller but the RL02's are powered down. 
I'll see if I can run it for a bit on RSX11M and get it to fault (and 
backup my disk first as it's nice to have it working again).


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