MM11-B core memory

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The MM11-B was a 8 Kbyte core main memory for the early PDP-11 UNIBUS machines. An MM11-B was composed of two hex boards, one piggy-backed on the other, and the pair taking only a single backplane slot (electrically):

  • An H221-D daughter-board containing the cores
  • A G651 mother-board containing most of the electronics, and the contact fingers for plugging into a backplane.

There was also a parity-capable variant, the MM11-BP, which uses an M7850 parity controller plugged into the same backplane as the MM11-B, and substituted a core array with two more bits per word, the H221-C.

It is possible to interleave a pair of MM11-B's to provide reduced effective average access times.

The MM11-B did not use a custom backplane; it plugged into a standard MUD slot. The pair was 'thick' enough that a normal board cannot be plugged into the next slot; instead, a G727 grant continuity card must be used there (since it has no components on it, it just clears the H221 card).


The DEC manuals for the MM11-B are not available online. Basic address configuration is by an 8-position DIP switch (in 4 groups of 2):

Address bit Switches
A17 S8, S7
A16 S6, S5
A15 S4, S3
A14 S2, S1

The jumpers used to configure the MM11-B address are:

Address A13 W8 W9
Lower 8KB In Out
Upper 8KB Out In

Jumpers W4 through W7 replicate the functionality of S1-S8. For interleave control:

Interleave W0 W1 W2 W3
Off In Out In Out
On Out In Out In

In interleaved memories, the S1 and S2 pair should be set to opposing settings in each of the two boards; i.e. both on in one, and both off on the other. (This does result in sequential locations in the interleaved banks being selected on A14, whereas jumpers W8/W9 select the low/high bank address, on A13; the latter cannot be used for interleave, though, as the board's wiring requires the use of A14 for that.)

They are on the G651 board, on the center right (with the board upright, with the contact fingers at the bottom). W4-W7 are in the center group there, interspersed with resistors; W0, W1, W3 and W2 are on the left end of the right-hand group (toward the edge of the board), and W8 and W9 are on on the right end.

See also