DEC PDP-11/45 backplane +5 ECO

[Noel Chiappa]

    > From: Henk Gooijen 

    > I have the M8120 and 4 M8121 boards (32kW bipolar RAM). It is a bit
    > weird, but in the 11/55 are also two G114 boards (4kW MOS RAM), IIRC.

G114s? Those are the sense/inhibit module from the MM11-U/MJ11. Did you
mean G401s?

If so, one guess as to what happened there is that the machine used to have
two banks of MS11 Fastbus memory, one bipolar, and one MOS, and some of the
boards from the MOS bank (the memory control, and maybe some of the matrix
boards) got removed?

A KB11-[AD] can have two banks of MS11; the only type mixing allowed is that
one can be all bipolar, and one all MOS; within each bank they all have to be
the same. More here:

  https://gunkies.org/wiki/MS11_Semiconductor_Memory_System

Interesting factoid: the M8110 and M8120 use the same etch. I'm not sure
quite what the difference is (the MS11-A MM doesn't say, I couldn't find, and
I don't think we have the M8120 engineering drawings, just the M8110);
the M8120 has a bunch of ECO wires on it, and maybe there are component changes
too. (I don't have an M8110 to compare them directly.)

	Noel