They came in several different generations: the earliest ones provided only quad Small Peripheral Controller slots; later ones also provided hex Modified UNIBUS Device slots (along with two SPC slots, in the end slots, where the UNIBUS entered and left the backplane on BC11A cables)).
Each slot held six 'connectors', DEC's term for a group of edge connector pins, denominated 'A'-'F'.
The earliest one, the DD11-A (used in the PDP-11/20) got its power through a paddle card which plugged into the 3A slot/connector. All the later ones used one or more standard DEC power distribution connectors.
- DD11-A backplane - 4 slot
- DD11-B backplane - 4 slot, with early power distribution connector
- DD11-C backplane - 4-slot MUD, with later power distribution connectors
- DD11-D backplane - 9-slot MUD
- DD11-P backplane - modified DD11-D used to hold the PDP-11/34 CPU
Other manufacturers, including Plessey, also produced UNIBUS backplanes, such as the Plessey PM-D11/SPC-1, a 9-slot unit.