SBus

From Wikipedia, the free encyclopedia

SBus
Four SBus slots
Year created:	1989
Created by:	Sun Microsystems
Superseded by:	PCI (1997)

Width:	32 bits
Number of devices:	8 masters, unlimited slaves
Capacity	16.6 MHz - 25 MHz
Style:	Parallel
Hotplugging?	no
External?	no

Two SBus cards

SBus male connector

SBus is a computer bus system that was used in most SPARC-based computers (including all SPARCstations) from Sun Microsystems and others during the 1990s. It was introduced by Sun in 1989 to be a high-speed bus counterpart to their high-speed SPARC processors, replacing the earlier (and by this time, outdated) VMEbus used in their Motorola 68020- and 68030-based systems and early SPARC boxes. When Sun moved to open the SPARC definition in the early 1990s, SBus was likewise standardized and became IEEE-1496. In 1997 Sun started to migrate away from SBus to PCI, and today SBus is no longer used.

SBus is in many ways a "clean" design. It was targeted only to be used with SPARC processors, so most cross-platform issues were not a consideration. SBus is based on a big-endian 32-bit address and data bus, can run at speeds ranging from 16.6 MHz to 25 MHz, and is capable of transferring up to 100 MB/s. Devices are each mapped onto a 28-bit address space (256 MB). Only eight masters are supported, although there can be an unlimited number of slaves.

When the 64-bit UltraSPARC was introduced, SBus was modified to use clock doubling and transfer two 32-bit data words per cycle to produce a 200 MB/s 64-bit bus. (For contrast, modern 66 MHz/64-bit PCI is 528 MB/s.) This variant of the SBus architecture used the same 96-pin connector as the older one.

SBus cards have a very compact form factor; a single-width card is 3 inches wide by 5 inches long and is designed to be mounted parallel to the motherboard. This allowed for three expansion slots in the slim "pizza box" enclosure of the SPARCstation 1. The design also allows for double- or triple-width cards that take up two or three slots, as well as double-height (two 3x5 inch boards mounted in a "sandwich" configuration) cards.

SBus was a peripheral interconnect only; some Sun systems used MBus, another standardized system, as a CPU-memory bus.