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:

  https://gunkies.org/wiki/KTJ11-B_UNIBUS_adapter#QBUS_slots

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
differently.


    > 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
failure).

	Noel


More information about the cctalk mailing list