Small Peripheral Controller

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Small Peripheral Controller or SPC was DEC's name for an I/O board slot in the backplanes of UNIBUS PDP-11s. It was a quad slot, occupying rows C-F in a hex slot, and could hold any kind of device.

It was originally conceived to hold a dual-height device-specific card, along with single-height M105 Address Selector and M782 (later M7820 and M7821 revisions) Interrupt Control FLIP CHIPs. 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 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.

SPC Extensions

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.

See Also