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 extra pins (in the A/B rows) to carry UNIBUS signals (pin-compatible with the usual dual UNIBUS slot), additional voltages (+ and 1 15V, -5V, and +20V), and a number of lines for parity communication between a parity controller, and memory boards plugged into that backplane.

MUD slots, like SPC slots, were wired to bring all 4 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, 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).

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.