MJ11 memory system

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The MJ11 memory system was a core main memory system for the PDP-11/70. It was 32 bits wide, to interface to the -11/70's Main Memory Bus, and provided byte parity (4 bits/double-word). Read, Write, and Exchange cycles were supported.

It came in two variants, the MJ11-A (which used 32KB core stack modules), and the MJ11-B (which used 64KB modules). A maximum of eight MJ11 systems could be installed on a single machine, for a maximum of 2MB of memory when using 32KB modules, or slightly less than 4MB with 64KB modules.


It was housed in a BA11 mounting box, and used a custom backplane. A backplane held a pair of control boards (an M8149 transceiver board, and either an M8147 or M8148 control and timing board; the M8148 could only be used with 32KB core stack modules, whereas the M8147 could be used with either), and between 1 and 4 pairs of core stack sets.

A 32KB core stack set consisted of:

  • G114 sense/inhibit module
  • G235 X-Y driver
  • H217-C core stack

A 64KB set consisted of:

  • G116 sense/inhibit module
  • G236 X-Y driver
  • H224-C core stack

The 32KB stack set contains the same boards as those in the MM11-U core memory; the 64KB stack set contains the same boards as those in the MM11-W core memory.

In the 64KB set, the H224-C is connected to the other two cards with 'over-the-back' cables; this is possibly because the MJ11 backplane was designed for use with the MM11-U board set, and changes in the functional modularization may have made those connections unsuitable for use by the 64KB set.

The H224-C core stack is connected to the G236 with a 20-pin cable, and to the G116 with three 40-pin connectors. The cable end on the G116/G236 is a standard Berg header, but on the H224-C end, the cables are soldered directly to the board, not plugged into a connector.

The stack sets were placed in the backplane from the outer edges (on each side, since they had to be installed in pairs of sets) toward the center; the control boards went in the two center slots. Interestingly, the order within each group of 3 boards was not mirror-imaged; the X-Y driver, core stack, and sense/inhibit module were in the same order on both sides of the center.


An MJ11-A could be upgraded to an MJ11-B by adding a pair of wires to the backplane (presumably for an extra address line), swapping in an M8147 control and timing board for the M8148, and swapping in, or adding, pairs of 64KB board sets. The wire ran from pin EJ1 of the sense/inhibit module to pin EJ2 of the X-Y driver board. (Later backplanes. not just those for the MJ11-B, came with this modification.)

When an MJ11 backplane contained a mix of 32KB and 64KB board sets, the 32KB ones had to be in the lower locations (i.e. the outer edges on each side). Both kinds of sets had to be installed in pairs; i.e. a 32KB set could not be paired with a 64KB set.

See also