Small Peripheral Controller
From Computer History Wiki
Small Peripheral Controller was DEC's name for an I/O board slot in the backplanes of UNIBUS PDP-11s. It was a quad slot, and could hold any kind of device.
SPC slots were wired to bring all 4 UNIBUS grant lines through the device; the board generally had a header which routed the grant (and matching request) line for the desired priority level to the on-board interrupt circuity, and passed the other grant lines through. The DMA (NPG) grant line generally had a jumper on the backplane, which had to be removed if a DMA device was plugged into the slot. Un-occupied slots needed to have a grant continuity card installed.
