SCSI Ultra SCSI Whitepaper What is SCSI

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.


    • Interface Technology BusWidth {bits} Clock Mhz ThroughPut MB/s Cable Pins Termination Length {Meters}
    Single SCSI-1 8 5 3 50 Passive 6
    Single SCSI-2 8 5 5 50 Active 6
    Single Fast-SCSI 8 10 10 50 Active 3
    Single Fast-Wide 16 10 20 68 Active 3
    Single Ultra 8 20 20 50 Active 1.5
    Single Ultra-Wide 16 20 40 68 Active 1.5
                   
    LVDS Ultra-2 8 40 40 50 Active 12
    LVDS Wide-Ultra2 16 40 80 68 Active 12
    LVDS Wide-Ultra3 16 80 160 68 Active 6
    LVDS Wide-Ultra160 16 40 160 68 Active 12