Using 1MB M8750 cards in a PDP-11/70

Noel Chiappa jnc at
Sun Jun 12 07:30:08 CDT 2016

So there was some prior discussion on the list about using the later 1MB
M8750 cards (used in the VAX-11/750, /730, etc) in the MK11 memory of a

However, that seems to require some kludgery in the MK11 (sending 4 separate
'board select' signals to a single memory card, etc).

I'm interested in using the 1MB M8750 cards because the smaller 256KB M8728
cards (which the MK11 _will_ take, although there's very little about that in
extant DEC documentation) are now seemingly unobtainium.

(Almost all the MK11 documentation that I have only talks about the 64K
M7984. The only one that does mention the M8728 is the -11/70 Maintenance
Service Guide, KA-K1170-MG-003, and it doesn't say much, just indicates that
one just plugs them in, and the controller recognizes them. So I've been
studying the prints to see how that all works - there are per-slot/board
'personality' lines the MK11 can read.)

The thing is that M8728 cards use the exact same PCB as the M7850 - in fact,
there are lots of 'M8728' cards listed online, because it says 'M8728' in the
etch, but they are in fact M8750's (as shown on the handles)!

So I had a brainwave: why not use M8750's, but jumper them to look like
M8728's? I'd only be using 256KB of a 1M card, but I can live with that (I
just bought a giant stack of M8750's for a fairly nominal amount). It would
let me use the M8750's without having to do hardware mods to my MK11.

So, has anyone here ever worked out how to use M8750 cards in the MK11 in this

It looks like all one has to do is i) figure out how to jumper the M8750 to
look like an M8728 (there is no documentation on the M8728 extant, AFAICT),
and ii) make sure that the A7 pin to the 64K RAM chips of the M8750 is driven
consistently to either '1' or '0' (it looks like there's a jumper in the A7
drive line, and one could connect the appropriate post of that jumper to
ground to do that).

So, if someone has done this before, or, failing that, if anyone has M8728
documentation (although I can get the M8728 jumper etc info off an M8728 I
have coming, if not), I would be extremely grateful!

Thanks in advance (I hope :-)!


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