MM11-U/UP core memory backplanes

william degnan billdegnan at
Fri Feb 26 14:49:42 CST 2016

On Fri, Feb 26, 2016 at 3:15 PM, Noel Chiappa <jnc at>

> So I have some MM11-U UNIBUS core memory cards, and in trying to locate a
> backplane to plug them into, I remembered a story I'd read on Guy's
> Web-site
> about some issues he'd had with parity and non-parity operation:
> and mention of the backplanes as being specific to parity and non-parity
> operation. So, I figured I'd better make sure what kind of MM11-U's I had,
> and get the correct backplane (MF11-U or MF11-UP). So I looked at the
> prints
> sets, and... the part number for the parity and non-parity backplanes is
> the
> same. WTF?
> Extensive further digging finally turned up this, in the "MF11-U/UP core
> memory system maintainence manual" (DEC-11-HMFMA-B-D, 3rd Printing,
> November
> 1974):
>     5.2.3 "Installation Procedure"
>     "A backplane jumper is required from B1U1 to B2U1 for non-parity
> memory.
>     This jumper should not be present for parity memory."
> Which makes sense. Apparently, for parity operation, SSYN is routed through
> the M7259 Parity Controller, and it only issues the SSYN once it has
> checked
> that the parity is OK. So, on non-parity installations, the 'proto-SSYN'
> from
> the rest of the memory is just hard-wired straight to the output to the
> UNIBUS (hence the jumper).
> Also, since the MM11-L system uses the same parity controller board
> (M7259),
> it seems a reasonable conclusion that there should be a similar jumper on
> the
> MF11-L backplane, and I have found such a jumper on some MF11-L backplanes,
> but I have yet to find anything in any DEC documentation about it.
> Speaking of MM11-L/ME11-L/MF11-L backplanes, there appear to be at least
> four
> different etch revisions (and I suspect the earliest one cannot be tweaked
> to
> run parity, since it _doesn't_ have that jumper on it). But that's for
> another
> day!
>         Noel

Very useful, thanks.  I will try this.
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