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 sections) 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; 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.