VAX-11/750 memory in 11/70 ?

Ethan Dicks ethan.dicks at
Tue Mar 10 17:39:09 CDT 2015

On Tue, Mar 10, 2015 at 5:40 PM, Pontus <pontus at> wrote:
> Hi all
> I was told that VAX-11/750 memory could also be used in an PDP-11/70 (or
> maybe some other model). Is this true?


> I have also read that there are three kinds of memory boards for the 11/750:
> M8728: 256k byte memory array;
> M8750: 1M byte memory array;
> M7199: 4M byte memory array.

Yes.  That sounds right (without looking up specific handle numbers),
depending on which memory controller you have in your 11/750.  In
mine, I can handle 256K and 1MB boards.  You also have to add row
wires to the backplane (I did this to upgrade a 2MB 11/750 to 8MB).  I
have never had the fortune to run an 11/750 with 4MB boards (max of
14MB mixed with 1MB boards due to max physical address space of 16MB,
shared with other resources).

> (As well as some third party boards.

Yes... CMI and others.  I've run a couple of those.  They might or
might not pass DEC diags but the OS doesn't care.

> Which one of these, if any, could be used in an 11/70?

I am aware of the 256K array board working in the 11/70.

Also, the 1MB board that works in the 11/750 also works in the 11/725
and 11/730.  I _think_ the 256K board should work in the 11/725 and
11/730 but since you are limited to 5 memory boards in that backplane,
1.25MB isn't enough to boot a VMS that's compatible with that CPU.  So
electrically it would work, but logically, you wouldn't be able to
boot it.  I suppose one could try 2x1MB+3x256KB and that might be
enough to boot an ancient VMS, but in practice, every 11/730 I've used
was crying for 5MB of physical RAM.


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