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Abbreviation of Small Computer System Interface. Pronounced "scuzzy," SCSI is a parallel interface standard used by Apple Macintosh computers, PCs, and many UNIX systems for attaching peripheral devices to computers. Nearly all Apple Macintosh computers, excluding only the earliest Macs and the recent iMac, come with a SCSI port for attaching devices such as disk drives and printers.

SCSI interfaces provide for faster data transmission rates (up to 80 megabytes per second) than standard serial and parallel ports. In addition, you can attach many devices to a single SCSI port, so that SCSI is really an I/O bus rather than simply an interface.

Although SCSI is an ANSI standard, there are many variations of it, so two SCSI interfaces may be incompatible. For example, SCSI supports several types of connectors.

While SCSI has been the standard interface for Macintoshes, the iMac comes with IDE, a less expensive interface, in which the controller is integrated into the disk or CD-ROM drive. Other interfaces supported by PCs include enhanced IDE and ESDI for mass storage devices, and Centronics for printers. You can, however, attach SCSI devices to a PC by inserting a SCSI board in one of the expansion slots. Many high-end new PCs come with SCSI built in. Note, however, that the lack of a single SCSI standard means that some devices may not work with some SCSI boards.

The following varieties of SCSI are currently implemented:

  • SCSI-1: Uses an 8-bit bus, and supports data rates of 4 MBps
  • SCSI-2: Same as SCSI-1, but uses a 50-pin connector instead of a 25-pin connector, and supports multiple devices. This is what most people mean when they refer to plain SCSI.
  • Wide SCSI: Uses a wider cable (168 cable lines to 68 pins) to support 16-bit transfers.
  • Fast SCSI: Uses an 8-bit bus, but doubles the clock rate to support data rates of 10 MBps.
  • Fast Wide SCSI: Uses a 16-bit bus and supports data rates of 20 MBps.
  • Ultra SCSI: Uses an 8-bit bus, and supports data rates of 20 MBps.
  • SCSI-3: Uses a 16-bit bus and supports data rates of 40 MBps. Also called Ultra Wide SCSI.
  • Ultra2 SCSI: Uses an 8-bit bus and supports data rates of 40 MBps.
  • Wide Ultra2 SCSI: Uses a 16-bit bus and supports data rates of 80 MBps.


    More Information

    Outstanding Page Adaptec's home page
    Home page of Adaptec, supplier of network, server, desktop, portable, chip solutions as well as FireWire host adapters. Their home page contains detailed product descriptions, company information, software demos and upgrades, drivers, a developer site, and a section on emerging technology. Also includes an Adaptec SCSI product selector. Updated on Aug 5, 1998

    Outstanding Page Enhanced IDE and SCSI White Paper
    Explores the differences between Enhanced IDE and SCSI disk drives with information on multitasking, data transfer and connectivity. Updated on May 22, 1998

    Outstanding Page Hard Disk Interfaces and Configuration
    Describes the different major interface standards currently used by hard disks (and other devices). Provides sections about the IDE/ATA and SCSI interfaces. This page is from "The PC Guide."

      A Visual Guide to SCSI Connectors
    Displays diagrams of common SCSI bus connectors.

      Article on the performance of SCSI and EIDE hard drives
    PC Lab article describing how SCSI drives perform relative to Enhanced IDE models. Updated on Jul 31, 1998

      Digital Interactive Solutions Limited
    Home page of Digital Interactive Solutions Limited, manufacturer of tape storage products and a range of modern tape drives and sub-systems. The site offers company and product information as well as technical reference information.

      Eric's Fast Graphics Site
    Contains overclocking information and PC cooling information, as well as game benchmarks, a repository of SCSI information, and various articles, reviews, and links. Updated on Dec 28, 1997

      SCSI diagnostic tools
    Describes CoComp's SCSI Pro - SCSI diagnostic utility. Updated on Feb 11, 1998

      SCSI Explorer
    A list of annotated links to all things retated to SCSI.

      SCSI FAQs
    Addresses questions and answers collected from the comp.periphs.scsi newsgroup.

      SCSI Just Keeps On Rolling
    PC Magazine article (March 1998) that examines the process by which the SCSI interface transfers data among SCSI devices. A sidebar explores the different types of SCSI and describes how SCSI works. Updated on Aug 4, 1998

      SCSI Trade Association Home Page
    Includes news items, white papers, articles and specifications, as well as an area with answers to technical questions about the SCSI interface.

      Small Computer Systems Interface (SCSI) Overview
    Describes SCSI standards, protocols and transfer modes, host adapters, and configuration and cabling. This page is from "The PC Guide."

      T10 Home Page
    Contains information about I/O Interfaces, especially SCSI, SCSI-2, and SCSI-3 including SPI, Fast-20 (Ultra SCSI), Fast-40 (Ultra2 SCSI), Low Voltage Differential (LVD), SPI-3 (Ultra3 SCSI or Ultra 160/m), and CAM. There are also links to sites on Fibre Channel, SSA, ATA (IDE), and ATAPI.

      Ultra SCSI White Paper
    Describes the history of SCSI and its evolution. Also compares Ultra SCSI with some alternatives, such as FC-AL and SSA. Updated on Jul 10, 1998

      Yahoo!'s SCSI page
    Yahoo!'s directory of SCSI. Updated on Aug 4, 1998

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