SBus

Four SBus connectors (top of photograph)
Year created	1989; 36 years ago (1989)
Created by	Sun Microsystems
Superseded by	PCI (1997)
Width in bits	32
No. of devices	8 masters, unlimited slaves
Speed	16.67 MHz - 25 MHz
Style	Parallel

SBus is acomputer bus system that was used in mostSPARC-based computers (including allSPARCstations) fromSun 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1989 to be a high-speed bus counterpart to their high-speed SPARC processors, replacing the earlier (and by this time, outdated)VMEbus used in theirMotorola68020- and68030-based systems and early SPARC boxes.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 thePeripheral Component Interconnect (PCI) bus, and today SBus is no longer used.^[1]

The industry's first third-party SBus cards were announced in 1989 by Antares Microsystems; these were a10BASE2 Ethernet controller, a SCSI-SNS host adapter, a parallel port, and an 8-channel serial controller.

The specification was published by Edward H. Frank and James D. Lyle.^[1]A technical guide to the bus was published in 1992 in book form by Lyle,^[2] who founded Troubador Technologies. Sun also published a set of books as a "developer's kit" to encourage third-party products.^[3]

At the peak of the market over 250 manufacturers were listed in the SBus Product Directory, which was renamed to the SPARC Product Directory in 1996.

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 abig-endian32-bit address and data bus, can run at speeds ranging from 16.67 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 the64-bitUltraSPARC was introduced, SBus was modified to support extended transfers of a 64 bits doubleword per cycle to produce a 200 MB/s 64-bit bus. This variant of the SBus architecture used the same form factor and was backward-compatible with existing devices, as extended transfers are an optional feature.

SBus cards had a very compact form factor for the time. A single-width card was 83.82 millimetres (3.300 in) wide by 146.7 millimetres (5.78 in) long and is designed to be mounted parallel to the motherboard. This allowed for three expansion slots in the slim "pizza box" enclosure of theSPARCstation 1.^[4] 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 originally announced as both asystem bus and a peripheral interconnect that allowed input and output devices relatively low latency access to memory.^[5] However, soon memory andcentral processing unit (CPU) speeds outpaced I/O performance. Within a year some Sun systems usedMBus, another interconnection standard, as a CPU—memory bus. The SBus served as an input/output bus for the rest of its lifetime.

• List of device bandwidths

• SBus Specification at Bitsavers
• 15205-2000 - ISO/IEC 15205:2000 (IEEE Std 1496-1993) SBus -- Chip and Module Interconnect Bus

• Computer buses
• Sun Microsystems hardware
• Motherboard expansion slot