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 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).

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.