Extended UNIBUS

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 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 rows to carry EUB signals (A18-A21, principally), thus making it not pin-compatible with the usual dual UNIBUS slot).

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 voltage of +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.

Pinout

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
• 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 requestts) 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.

Pins

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)