MM11-U/UP core memory backplanes

[Noel Chiappa]

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:

  http://www.shiresoft.com/pdp-11/11-10/index.html

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