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Last modified: Monday, September 20, 2004 

(sku´zē) Short for small computer system interface, 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.
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    For pages about SCSI . Also check out the following links!

    Related Links

    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."

    Storage Basics -- SCSI Part I 
    This article looks at SCSI as a technology, discusses some of the more popular SCSI standards and examines SCSI device numbering.

    Storage Basics -- SCSI Part II 
    This article focuses on SCSI implementation considerations such as signaling, termination and connector types.

    All About SCSI
    Provides an introduction to SCSI and discusses SCSI evolution, standards, termination, and configuration.

    Fast Hard Drives Page
    This user page contains a collection of links to benchmark resouces, controllers, disks, distributors, newsgroups, vendor white papers, and reviews and FAQs for hard drives.

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

    SCSI Library
    News, technical information, FAQs regarding SCSI.

    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."

    What Kind of SCSI Do I Have?
    Helps users determine if they have single-ended SCSI or a differential SCSI interface.

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    QVS: SCSI Switches & Cables - Manufactures cables and connectivity products for the computer and communication industries. SCSI cables double shielded to support maximum speed.

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    L-com Connectivity: SCSI Adapters and Terminators - Factory direct online merchant offering cabling, connectors, and network equipment, including SCSI adapters and terminators.

    related categories


    Miscellaneous Standards

    Peripheral Devices

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