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).

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.

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).

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.

Pinout

The pins which changed from MUD to EUB are:

• AB1 - Reserved
• AB2 - Boot Enable
• AN1 - A21
• AP1 - A20
• AP2 - Reserved
• AR1 - +15V/12V Battery
• AR2 - Reserved
• AS1 - +15V/12V Battery
• AS2 - Reserved
• AU1 - Reserved
• AU2 - Reserved
• AV1 - Reserved
• AV2 - Reserved
• BA1 - Reserved
• BB1 - +5V Battery
• BB2 - Reserved
• BD1 - +5V Battery
• BE1 - A19
• BE2 - A18
• BV2 - Reserved