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

Noel Chiappa jnc at mercury.lcs.mit.edu
Wed Apr 22 10:59:01 CDT 2020

    > From: Mark Matlock

    > Are you able to use a Qbus MTI controller in the 11/84's Qbus section
    > of the backplane? This is something I've often wondered about but never
    > tried.

I looked into this in some detail, but I don't know:


Amswering it definitively would probably require looking at DMA and interrupt cycles
with a logic analyzer on the bus, to see what the CPU does with them. And the KDJ11-B
and KDJ11-E (the 11/94 uses the same bacplane with a different CPU card) might act

    > For anyone who is curious about what Happens when a M8637-C version
    > board is used as PMI memory in an 11/83 I can speak From experience.
    > ...
    > After this the disk is corrupted and you will need to restore the
    > system disk from backups

Ah, very informative; thanks for reporting.

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


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