Differences between PDP-11/83-84 M8190-AB/AC and AD/AE CPU cards

Noel Chiappa jnc at mercury.lcs.mit.edu
Sun Feb 8 08:58:03 CST 2015

    > From: Jerome H. Fine

    > for the PDP-11/83 (usually in a BA23 or a BA123), the backplane is
    > wired so that the PMI memory is activated correctly WHEN the first
    > memory board is in slot 1 and the CPU follows the memory boards.

That's an artifact of the way Qxx/CD backplanes, and PMI CPU cards, are
wired. (And you probably already knew this, but just to be explicit... :-)

For the backplanes, with few exceptions, pin nQm2 (where n is the slot #, Q
is C or D, m is A-V, in the DEC alphabet) is wired only to pin (n+1)Qm1 -
i.e. pin Qm1 is not wired through to all slots. In other words, the CD bus is
not carried _through_ a slot, only _between_ slots.

The PMI bus on the CPU is only wired to the top (i.e. 1) pins. So if you plug
a PMI CPU into a normal Q22/CD backplane, the only slot that receives the PMI
signals is the slot _before_ the CPU. Hence the requirement to plug memory
cards in _before_ the CPU when using a Q22/CD backplane for PMI.

The 11/84 backplane is different; per the backplane prints in
EK-PDP84-TM-PR2, the 1-side pins on the Q22/'CD' portion of the backplane
_are_ all bussed together.

(Note: from what I can see in the backplane prints in EK-PDP84-TM-PR2, none
of the other CD bus pins are bussed together; in particular, the staggered
connections from nCA1 to n+1CC1, nDA1 to n+1DC1, etc found in the real Qnn/CD
backplane don't seem to be there in the 11/84 backplane. So it's not _real_
Q22/CD - it's probably better to call that section of the 11/84 backplane
Q22/PMI. PMI _can_ run over a normal Q22/CD backplane, though.)

    >> Would a KDJ11-A board even work in an 11/84 backplane?

    > I meant the KDJ11-BD board or the M8190-AC.
    > ...
    > it seems reasonable to hope that the PDP-11/84 can also use the
    > KDJ11-BD instead of the KDJ11-BF

Ah. My guess (having not looked at the 8190-AC in detail) is that to the
extent the M8190-AC implements PMI, it should work fine in an 11/84.


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