Unibus
| Unibus | |
Unibus backplane (left) and two printed circuit boards
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| Year created | 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.
The Unibus consists of 72 signals, usually connected via two 36-way edge connectors on each printed circuit board. When not counting the power and ground lines, it is usually referred to as a 56-line bus. It can exist within a backplane or on a cable. Up to 20 nodes (devices) can be connected to a single Unibus segment; additional segments can be connected via a bus repeater.
The bus is completely asynchronous, allowing a mixture of fast and slow devices. It allows the overlapping of arbitration (selection of the next bus master) while the current bus master is still performing data transfers. The 18 address lines allow the addressing of a maximum of 256 kiB. Typically, the top 8 kiBw is reserved for the registers of the memory-mapped I/O devices used in the PDP-11 architecture.
The design deliberately minimizes the amount of redundant logic required in the system. For example, a system always contains more slave devices than master devices so most of the complex logic required to implement asynchronous data transfers is forced into the relatively few master devices. For interrupts, only the interrupt-fielding processor needs to contain the complex timing logic. The end result is that most I/O controllers can be implemented with simple logic, and most of the critical logic is implemented as a custom MSI IC.
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