SCSI Knowledge Center


*What is SCSI?

SCSI (Small Computer System Interface) technology allows you to attach various internal and external devices to a single controller card in your computer. SCSI hardware is often used in high-end PCs and server computers because of its greater flexibility and faster performance when compared to other Input/Output (I/O) technologies.

A typical SCSI configuration consists of the following items:
  • A SCSI host adapter (or controller) that plugs into a slot in your computer's motherboard.
  • Internal and/or external SCSI devices (such as hard drives, CD-ROM, DVD, scanner).
  • Internal and/or external SCSI cables, SCSI terminators, and possibly SCSI cable adapters.

    Careful selection of SCSI hardware is necessary to ensure proper operation. The following information should help you get started in configuring your SCSI-enabled computer. You should also check your SCSI device documentation for special requirements.

    General SCSI Specifications

    Bus Width SCSI Standard Maximum Speed Maximum Length Common Connectors
    8-bit
    (Narrow)
    SCSI-1 5 MB/s 6m
    (19.6 ft.)
    Ribbon IDC 50

    Internal 50-pin connector for hard drives, CD drives, etc.

    Centronics 50

    Popular 50-contact connector for external peripherals such as scanners and tape drives.

    DB25

    External 25-pin connector used by Zip drives, scanners, and many Mac devices.

    DB50

    External 50-pin connector used by Sun workstations.

    SCSI-2
    Fast SCSI,
    Fast-10
    10 MB/s 3m
    (9.8 ft.)
    Ribbon IDC 50

    Internal 50-pin connector for hard drives, CD drives, etc.

    Half Pitch DB50

    External 50-pin host and peripheral connector used by Jaz drives and many CD drives.

    Ultra SCSI,
    Fast-20
    20 MB/s 1.5m
    (4.9 ft.)
    Ultra2 SCSI 40 MB/s 12m
    (39 ft.)
    16-bit
    (Wide)
    SCSI-2,
    Fast Wide SCSI, 
    Fast-10
    20 MB/s 1.5m
    (4.9 ft.)
    Half Pitch DB68

    Internal and external 68-pin connector used by most new hard drives.  The external version includes thumbscrews or latches.
    Ultra Wide SCSI,
    Fast-20
    40 MB/s 1.5m
    (4.9 ft.)
    SCSI-3,
    Ultra2 Wide (U2W) SCSI,
    Ultra2 LVD SCSI,
    Fast-40
    80 MB/s 12m
    (39 ft.)
    Half Pitch DB68
    Low Voltage Differential (LVD) cables and accessories are required to achieve peak performance with U2W drives.

    Very High Density Centronics 68

    68-pin contact connector used on many servers & RAID controllers.

    SCA 80

    80-contact connector used by SCA drives for RAID controllers
    Ultra3 SCSI,
    Ultra160/m
    160 MB/s 12m
    (39 ft.) 


    * Maximum SCSI bus length is 25m (82ft.) if all devices are HVD (Differential).

    Please note that there is a difference between the terminology used and the physical characteristics of the cable. in "Wide SCSI" & "Ultra Wide SCSI" , "Wide" refers to the data path--a 68-pin cable has 16 bits of data. In fact, for external SCSI cable, a Centronics 50-pin "Narrow SCSI" connector is physically wider than Half Pitch 68-pin "Wide SCSI" connector. For internal SCSI cable, 50-pin Narrow SCSI cable is physically wider than 68-pin Wide SCSI cable.

    *Types of SCSI Signals

    There are three distinct types of SCSI signals:

    1.Single Ended (SE)
    Most SCSI devices use "normal" SE signaling, which limits the maximum length of a SCSI bus to 1.5m (4.9ft). This includes most 50-pin (Narrow) SCSI devices such as scanners and Zip drives.

    2. High Voltage Differential (HVD) (also known as "Differential")
    A Differential SCSI system provides reliable signaling in high noise environments over a long bus length (up to 25m [82ft]). HVD hardware cannot be mixed with other SCSI signal types. A Differential bus requires Differential terminators.

    3. Low Voltage Differential (LVD)
    A typical multimode LVD/SE SCSI system provides a moderately long bus length (up to 12m [39ft]) and downward compatibility with SE hardware. LVD-rated equipment is required for "Ultra" SCSI standards

    *Rules for Connecting SCSI Devices

  • SCSI is a "bus" architecture, which means devices are connected along a line that has a beginning and an end. This cabling scheme is commonly called a "daisy chain".
  • The two extreme ends of a SCSI bus segment must be properly "terminated". A terminator is a small device designed to dampen electrical signals reflected from the ends of a cable. Termination is disabled for any SCSI device that is positioned between the two ends.
  • The SCSI host adapter (or controller) may be positioned anywhere along the bus. It must be terminated if it is positioned at the end of the bus. Many host adapters provide automatic termination.
  • 8-bit (Narrow) SCSI devices require 50-pin (or fewer) connections. Up to 7 different devices may be controlled in a Narrow bus. Narrow SCSI signals include Low Byte & Control.
  • 16-bit (Wide) SCSI devices require 68-pin connections. Up to 15 different devices may be controlled in a Wide bus. Wide SCSI signals include Low Byte & Control, and High Byte.
  • SCSI is a downwards-compatible technology. Older SCSI devices can be installed in a newer (and faster) SCSI bus segment, but overall system performance may be reduced.
  • Each device on a SCSI bus must be assigned a unique identifier (SCSI ID) which is usually set by jumper caps. The ID is independent of the device's relative position on the bus. When mixing Narrow and Wide devices on the same bus, the ID of the host adapter should be set to 7 (or less) so that the Narrow devices can recognize it.
  • 50-pin to 68-pin SCSI cable adapters usually allow Narrow devices to be used in a Wide bus. Termination of the High Byte signal must be considered if a Narrow SCSI device is positioned at the end of a Wide SCSI bus.
  • 68-pin to 50-pin SCSI cable adapters usually allow Wide devices to be used in a Narrow bus. The speed of the Wide SCSI device will be limited to that of the Narrow SCSI bus.
  • High Voltage Differential (HVD) (also known as "Differential") SCSI drives and controllers should not be mixed with other SCSI hardware. A Differential bus requires Differential terminators.
  • Low Voltage Differential (LVD) SCSI devices require LVD cables and terminators. LVD SCSI hardware should not be confused with "Differential" hardware.
  • Most SCSI devices are Single-Ended (SE), which means they use "normal" electrical signals (not HVD or LVD as described previously).

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    SCSI Connectors:

  • Internal 50-pin connector for hard drives, CD drives, etc.
    Popular 50-contact connector for external peripherals such as scanners and tape drives.
    External 25-pin connector used by Zip drives, scanners, and many Mac devices
    External 50-pin host and peripheral connector used by Jaz drives and many CD drives.
    Internal and external 68-pin connector used by most new hard drives. The external version includes thumbscrews or latches
    68-pin contact connector used on many servers & RAID controllers
    80-contact connector used by SCA drives for RAID controllers
    Sun Sparcstation SCSI
    Mac Powerbook
    IBM PS/2; IBM RS/6000






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