Unibus

Unibus

Unibus
Unibus backplane (left) and two expansion cards
Year created	1969; 55 years ago (1969)
Created by	Digital Equipment Corporation
Width in bits	18 address, 16 data
Style	Parallel

The Unibus was the earliest of several computer bus and backplane designs used with PDP-11 and early VAX systems manufactured by the Digital Equipment Corporation (DEC) of Maynard, Massachusetts. The Unibus was developed around 1969 by Gordon Bell and student Harold McFarland while at Carnegie Mellon University.^[1]

The name refers to the unified nature of the bus; Unibus was used both as a system bus allowing the central processing unit to communicate with main memory, as well as a peripheral bus, allowing peripherals to send and receive data. Unifying these formerly separate busses allowed external devices to easily perform direct memory access (DMA) and made the construction of device drivers easier as control and data exchange was all handled through memory-mapped I/O.^[2]

Unibus was physically large, which led to the introduction of Q-bus, which multiplexed some signals to reduce pin count. Higher performance PDP systems used Fastbus, essentially two Unibusses in one. The system was later supplanted by Massbus, a dedicated I/O bus introduced on the VAX and late-model PDP-11s.