Modified UNIBUS Device

Modified UNIBUS Device or MUD was DEC's name for an I/O board slot in the backplanes of UNIBUS PDP-11s. It was a hex slot, and could hold any kind of device.

It differed from the older Small Peripheral Controller slot in that it was a hex-height slot, and used the pins (in the A-B connectors) to carry UNIBUS signals (mostly pin-compatible with the usual dual UNIBUS slot).

There were a few differences with the UNIBUS dual connector. The most important was that since the standard UNIBUS dual connector only contains one pin per grant (suitable only for an 'in' or 'out' UNIBUS connector), not separate 'grant in' and 'grant out' pins (as on the QBUS), the MUD pins do not contain grants.

The grant pins were recycled to contain a number of lines for communication between a parity controller, and parity-capable memory boards plugged into that backplane; and also additional voltages (primarily for core memory).

Technically, a MUD slot only describes the A-B connectors; most backplanes also provided SPC functionality in the C-F conectors (needed for grants, for devices which did interrupts or DMA), with most UNIBUS signals present in both; the combination is sometimes called MUD/SPC.

The dd11-c (4-slot0 and DD11-D (9-slot) and later backplanes generally provided MUD slots, not the earlier SPC slots; however, generally the slots in these backplanes are MUD/SPC - other than the entrance and exit slots, which have normal two-slot UNIBUS in A-B.

Grants

MUD/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), not the 'pseudo-UNIBUS' rows A/B,

A board plugged into a MUD slot 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.

Added signals

The added lines for communication between a parity controller and parity-capable memory boards were Parity Detect (used to let memory boards know that a parity controller is present); Internal SSYN (used by memory boards to let the parity controller know that their data is ready); and Parity P0 and Parity P1 (parity data).

The additional voltages were + and -15V, -5V, and +20V.

Pinout

The pins which changed from standard UNIBUS to MUD are:

• AB2 - Test Point
• AN1 - Parity P1
• AP1 - Parity P0
• AR1 - -15/12 Battery
• AS1 - -15/12 Battery
• AU1 - +20V (core)
• AV1 - +20V (core)
• AV2 - +20V (core)
• BA1 - Reserved
• BB1 - Reserved
• BB2 - Test Point
• BD1 - +5V Battery
• BE1 - Internal SSYN
• BE2 - Parity Detect
• BV2 - -5V (core)