Extended UNIBUS

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The Extended UNIBUS or EUB was DEC's name for both an upgrade to their standard UNIBUS PDP-11 bus, to add additional address lines, in order to support larger memories; and also for the slots in a backplane which instantiated the bus. The added address lines are A18-A21.

The slots were hex slot, and could hold only special EUB main memory cards, the only EUB cards ever produced. The only machines to support the EUB were the PDP-11/44 and PDP-11/24, which had a limited number of EUB slots in their main backplanes (the ones which held their CPUs).

EUB slots differed from the older MUD slots in that they re-purposed some of the pins in the A-B connectors to carry EUB signals (A18-A21, principally), thus making it not pin-compatible with the usual dual UNIBUS slot (which usually appears on the A-B connectors in such slots).

In the -11/24, at least, the EUB slots can also support a normal SPC device, in the C-F connectors usually used for SPC devices in hex slots. However, the two busses in such slots are not fully connected, as they are in MUD/SPC backplanes; instead, they form two somewhat separate busses. (See the PDP-11/24 article for the details.)

EUB memory boards

The only EUB-capable memories which DEC produced were the MS11-L, MS11-M, and MS11-P. The first two were jumperable for use on either the UNIBUS or EUB, although the second used the unusual power voltages of +12V/-12V, which most backplanes did not supply.

Other manufacturers also produced EUB memories.

Added signals

In addition to the extra address lines, power pins useful for battery backup of semi-conductor memory contents were added.


The pins which were added (changed) from MUD to EUB are:

  • AB1 - Reserved
  • AB2 - Boot Enable
  • AN1 - A21
  • AP1 - A20
  • AP2 - Reserved
  • AR1 - +15V/12V Battery
  • AR2 - Reserved
  • AS2 - Reserved
  • AT2 - Reserved
  • AU1 - Reserved
  • AU2 - Reserved
  • AV1 - Reserved
  • AV2 - Reserved
  • BB1 - +5V Battery
  • BB2 - Reserved
  • BD2 - Reserved
  • BE1 - A19
  • BE2 - A18
  • BV2 - Reserved

Removed signals

To provide the extra pins needed, since grants has already been removed, other interrupt-related signals (including requests) were removed. In addition, power pins for voltages which were no longer common (due to the replacement of core memory by semi-conductor memory) were also removed.


The pins which changed function from the MUD slot are listed below. Pins marked with a '*' were the ones which had retained their function from UNIBUS to MUD; the others had been already modified in that stage.

  • AB1* - INTR
  • AB2 - Test Point
  • AN1 - Parity P1
  • AP1 - Parity P0
  • AP2* - BBSY
  • AR1 - -15/12V Battery
  • AR2* - SACK
  • AS2* - NPR
  • AT2* - BR7
  • AU1 - +20V (core)
  • AU2* - BR6
  • AV1 - +20V (core)
  • AV2 - +20V (core)
  • BB1 - Reserved
  • BB2 - Test Point
  • BD2* - BR4
  • BE1 - Internal SSYN
  • BE2 - Parity Detect
  • BV2 - -5V (core)

See also