Small Peripheral Controller
It was originally conceived to hold a dual-height device-specific card, along with single-height M105 Address Selector and M782 Interrupt Control FLIP CHIPs (later, the M7820 and M7821 revisions). The appropriate UNIBUS signal lines (address, data, etc) were thus wired to the appropriate rows/pins in SPC slots. Other pins were wired to allow the necessary communication between the cards, without requiring cables between them.
It soon became more cost-effective to fabricate an entire device on a single quad card, but the pinout was retained. (For the pinout of an SPC slot, see here.)
SPC slots were wired to bring all 5 UNIBUS grant lines through the device; this was performed in rows C (for NPG) and D (for BGx). The device board generally had a header which routed the grant (and matching request) line for the desired priority level to the interrupt circuitry, 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 either a G727 or a G7273 grant continuity card installed.
On some systems, some SPC pins were recycled for other purposes.
In the PDP-11/04 and PDP-11/34, on the backplane which holds the CPU card(s), along with the KY11-LB Programmer's Console, the CPU and the Programmer's Console do some communication via the backplane. Pins CP1 and CR1 are Halt Request and Halt Grant, respectively; they allow the KY11-B to tell the CPU to halt.