Private Memory Interconnect

The Private Memory Interconnect (PMI) bus was a high-performance memory bus, a variant of the QBUS, introduced with the KDJ11-B PDP-11 CPU. It also provides means for the CPU and a KTJ11-B UNIBUS adapter to communicate in providing UNIBUS service.

For data transfer, PMI provides two primary modes; i) single- and double-word data reads, and single-word and single-byte reads; ii) block mode, which can read up to 16 words. When operating with a KTJ11-B, it provides means for UNIBUS devices to perform DMA cycles (mapped to the full 22-bit address space via a UNIBUS map), and to interrupt the CPU.

The PMI uses the CD interconnect specified for QBUS backplanes to carry PMI-specific signals; PMI also uses other existing QBUS signals (e.g. BDAL00-21), on their standard pins. Use of the standard CD interconnect means that PMI-systems (such as a PDP-11/83 can be constructed with a standard Q/CD backplane, a KDJ11-B CPU card, and one or more PMI memories.

Pinout

PMI pins are identified in the standard UNIBUS manner; there are four connectors, A-B; pins on the component side are 1, those on the solder side are 2. Pins are identified by the 'DEC alphabet', A-V, with G, I, O and Q dropped.

The tables below show the pins used for normal PMI master-slave cycles (as in the PDP-11/83), and those used to communicate with the KTJ11-B UNIBUS adapter (PDP-11/84 and PDP-11/94 only).

Master-slave signals

CE1 || PBCYC || Indicates a PMI cycle CR1 || PBSY || PMI busy DC1 || PBYT || Used with BWTBT to indicate type of PMI cycle CP1 || PBLKM || Indicates a PMI block mode transfer (i.e. multi-word) CB1 || PSSEL || Slave select DB1 || PWTSTB || Write strobe CJ1 || PSBFUL || Slave buffer full CM1 || PRDSTB || Read strobe CH1 || PHBPAR || High byte parity CK1 || PLBPAR || Low byte parity

Pin	Signal	Meaning

UNIBUS adapter communication signals

CF1 || PUBSYS || UNIBUS adapter present DD1 || PMAPE || Enable UNIBUS map for UNIBUS->PMI CD1 || PUBMEM || CPU access to UNIBUS CV1 || PUBTMO || UNIBUS timeout

Pin	Signal	Meaning