MM11-L core memory

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The MM11-L core memory was a popular, and common, 16 Kbyte core main memory unit for the early PDP-11 UNIBUS machines. An MM11-L was composed of a three board set:

  • G110 - hex-width memory control logic and data channels
  • G231 - hex-width memory driver logic
  • H214 - quad-width core stack

The MM11-L required a custom backplane, although some CPU's (e.g. the PDP-11/05) had processor backplanes wired to hold one or more MM11-L sets, as well as the CPU.

The MM11-S was a four-slot backplane plus a single MM11-L board set. The MF11-L was a nine-slot backplane plus a single MM11-L board set; the backplane has room for two more MM11-L board sets. The ME11-L was an MF11-L in a 5-1/4" rack-mount box, with power supply (the -LA was wired for 115V supply, the -LB for 230V).


There was also a parity variant, the MM11-LP, which added a dual-width M7259 Parity Control Module, and substituted an H215 (with two more bits per word) for the H214, and a G109 for the G110. Enabling parity increased the access time from 400 nsec to 525 nsec.

Parity versions (denoted by a 'P' suffix) were available for all of the above. The manual indicates that an 'MF11-LP backplane' was required, but it is not stated if this version had a different PCB, or if the non-parity version simply had a jumper, like the MM11-U core memory.

After some investigation, apparently there were two versions of the backplane, the early one (part #5409959) which cannot run parity, and a later one (part #5410331) which can. An example of the later backplane had the same jumper as that called for for the MM11-U, from pin 1BU1 to 2BU1. This jumper should not be present for parity memory.

(The parity controller is not in either slot 1 or 2; rather, the backplane wires together all the BU1 SSYN pins on the interior slots (2-8); the jumper connects together that line, and the main UNIBUS SSYN. The G109/G110 modules drive that 'internal' SSYN; hence the need to install the jumper for non-parity memory. When parity is enabled, the parity controller drives the bus SSYN via pin BV2 in its slot, which is directly connected to bus SSYN; the controller uses the 'internal' SSYN as input.)

Note that therefore parity and non-parity memory cannot be mixed on the same backplane. There is also no provision for use of the M7259 parity controller in the PDP-11/05 backplanes which support the MM11-L.

(One could probably easily ECO the earlier version to support parity, since the SSYN trace is accessible: cut the trace at 1BU1 and 9BU1, and add the needed wire-wrap lines.)

Module chart

The board organization in the MM11-L backplanes is not given in the manual; that for the MF11-L is:

Slot A B C D E F
1 UNIBUS In H213/H214 Core stack
2 G231 Memory Driver
3 G110 Memory Control
4 G231 Memory Driver
5 G110 Memory Control
6 M7259 Parity Controller H213/H214 Core stack
7 G231 Memory Driver
8 G110 Memory Control
9 UNIBUS Out H213/H214 Core stack

The first MM11-L set goes in slots 1/2/3; as MM11-L sets were added, they were installed in slot numerical order. (The location of the parity controller is the only un-used dual slot in a full MF11-L.)