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SCSI. Chapter 13. SCSI. Small Computer System Interface introduced in 1979 as a means of mass storage Common SCSI devices Hard drives- Scanners Tape backup units- CD-ROM drives Removable hard drives- Printers. SCSI.

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Chapter 13



Small Computer System Interface

  • introduced in 1979 as a means of mass storage

  • Common SCSI devices

    • Hard drives- Scanners

    • Tape backup units- CD-ROM drives

    • Removable hard drives- Printers

  • Scsi


    • SCSI manifests itself through a SCSI chain, which is a series of SCSI devices working together through a host adapter

    • Host adapter attaches to the PC

      • picture on page 866

    Scsi chains

    SCSI Chains

    • All SCSI devices can de divided into 2 groups: external and internal

    • External devices stand alone and hooked to the external connector of the host adapter

    • Internal devices attach to the host adapter internally and the devices sit in the PC

    • SCSI has 50-pin, 68-pin, or 80-pin cables

    Scsi chains1

    SCSI Chains

    • If the SCI cables are plugged in incorrectly, they can damage the devices

    • External devices all have 2 connections in the back, which enable you to daisy-chain

    • SCSI can have a maximum of 8 (the host plus 7 others) or 16 (host + 15) depending on the SCSI chain

    Scsi ids

    SCSI IDs

    • The values of the ID numbers range from 0 to 7, or 0 to 15

    • No 2 devices can share the same ID number

    • Host adapter is usually set to 7

    • Doesn’t matter what ID # other devices are set to

    • Try to use ID #0 for bootable hard drive

    • You can set SCSI IDs by setting jumpers

    Scsi chains2

    SCSI chains

    • Each end of the chain must be terminated

      • examples of termination on page 877

    • some devices self-terminate, but others you have to do yourself

    • usually set with jumpers; sometimes switches

    • can damage devices if terminating improperly

    Scsi flavors

    SCSI Flavors

    • Types of SCSI:

      • SCSI - 1: 8-bit

      • SCSI - 2: 8-bit

      • SCSI - 2: 16-bit (wide)

      • SCSI - 2: 32-bit (wide)

      • SCSI - 3: numerous

    • Refer to chart on page 884 for types

    Bus mastering

    Bus mastering

    • Why is bus mastering an important part of SCSI?

      • devices that are on a SCSI chain can communicate with each other with very little use of the CPU, which frees up the system resources

    Scsi cable and connectors

    SCSI Cable and connectors

    • SCSI-1 & SCSI-2 (8-bit) used Type A: 50 pins

    • SCSI-2 (16-bit) used Type B: 68 pins

    • Some of the higher end SCSI-3 cables use 80 pins



    • Advanced SCSI programmer interface

      • mandates a standard way to write BIOS device drivers for SCSI devices

      • you can install a standardized set of device drivers for all your SCSI devices

    Compatibility with ide

    Compatibility with IDE

    • IDE and SCSI can both exist in one machine

    • IDE will get priority for the boot drive; you should use CMOS setup which allows SCSI to take priority

    • Not a good idea to mix them



    • SCSI adapters need an IRQ and usually take 3, so be careful with conflicts with COM ports.

      • Usually only happens with legacy PCs

    • Read the cost and benefits section on 893

    Scsi vs ide

    SCSI vs. IDE

    • SCSI used to be much bigger, now the advantage is disappearing

    • SCSI is much faster, uses bus mastering, and can support up to 15 devices now but also more expensive

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