SCSI

SCSI
Name	Small Computer System Interface
Caption	A typical 50-pin SCSI-1 connector
Inventor	Larry Boucher
Developer	American National Standards Institute
Date	1986
Superseded by	Serial Attached SCSI, Fibre Channel

SCSI. The Small Computer System Interface is a set of standards for physically connecting and transferring data between computers and peripheral devices. Originally developed for hard disk drives, the protocol expanded to support a wide array of devices, becoming a mainstay in servers, workstations, and high-performance personal computers. Its command architecture allows for sophisticated control of multiple devices on a single bus, distinguishing it from simpler interfaces.

Overview

The standard was conceived in the late 1970s by Larry Boucher while working at Shugart Associates, which later became part of Seagate Technology. It was formally standardized in 1986 by the American National Standards Institute under the designation X3.131-1986. This standardization effort was crucial for fostering interoperability among different manufacturers like IBM, Apple Inc., and Compaq, who adopted it for their systems. The interface's ability to chain multiple devices, including scanners and tape drives, made it a versatile solution beyond simple storage.

Technical specifications

The original specification, now called SCSI-1, defined an 8-bit parallel bus with a 5 MB/s transfer rate and a maximum cable length of six meters. Key electrical signaling methods included single-ended and differential, with the latter offering greater noise immunity for longer distances. The protocol uses a sophisticated arbitration system allowing any device on the bus to initiate communication, managed by a host adapter often referred to as a SCSI initiator. Termination with a resistor pack at both ends of the bus is required to prevent signal reflections, a common configuration challenge.

Command set

At its core, the protocol defines a comprehensive command architecture that is transport-independent. The primary command set for block devices is the SCSI Block Commands, which handle operations for direct-access devices like hard drives. Another major set, the SCSI Stream Commands, governs sequential-access devices such as tape drives. This layered design separates the logical commands from the physical interface, a principle that allowed the command sets to be used over newer transports like Internet SCSI and Serial Attached SCSI.

Interface evolution

The parallel bus evolved through several generations to increase speed and reliability. SCSI-2, standardized in 1994, introduced Fast and Wide variants, doubling both clock rate and data bus width. The subsequent Ultra SCSI and Ultra-2 SCSI specifications pushed speeds to 40 MB/s and introduced Low Voltage Differential Signaling. The final major parallel iteration, Ultra-320 SCSI, achieved 320 MB/s. This evolution was driven by the needs of the enterprise storage market and companies like Adaptec, a major producer of host bus adapters. The parallel bus was ultimately supplanted by serial point-to-point interfaces.

Device types and applications

Beyond hard drives, the standard supported a diverse ecosystem of peripherals. It was the preferred interface for high-performance RAID arrays and optical jukeboxes in data centers. In creative fields, it connected flatbed scanners from UMAX Technologies and film scanners for professional digitization. The RISC-based NeXTcube and many Sun Microsystems workstations used it as their primary storage bus. Even early CD-ROM drives and magneto-optical drives often featured SCSI interfaces for their superior performance compared to ATA.

Comparison with other interfaces

For much of the 1990s, it competed directly with the Parallel ATA interface in the personal computer market. While PATA was simpler and cheaper, SCSI offered higher performance, better multitasking, and support for more devices per channel, making it dominant in servers. The introduction of Serial ATA in the early 2000s closed much of the performance gap for client systems. In the enterprise space, SCSI's successor, Serial Attached SCSI, now competes with Non-Volatile Memory Express over PCI Express and Fibre Channel for storage area networks. The resilience of its command set is evidenced by its use in the Universal Flash Storage standard.

Category:Computer buses Category:Computer hardware standards Category:Data transmission