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Chapter 8. All About SCSI. You Will Learn…. About basics of SCSI (Small Computer System Interface) technology and components How SCSI hard drives compare to IDE drives How to install a SCSI device Some troubleshooting tips for SCSI. SCSI Basics.

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Chapter 8

Chapter 8

All About SCSI

You will learn
You Will Learn…

  • About basics of SCSI (Small Computer System Interface) technology and components

  • How SCSI hard drives compare to IDE drives

  • How to install a SCSI device

  • Some troubleshooting tips for SCSI

Scsi basics
SCSI Basics

  • Standard for communication between subsystem of peripheral devices and system bus

  • Bus can contain/be used by up to 7 or 15 devices, depending on SCSI standard

  • Bus controller can be either:

    • An expansion card (host adapter)

    • Embedded on motherboard

  • Faster, more expensive, more difficult to install than similar IDE devices

The scsi subsystem
The SCSI Subsystem

  • Host adapter

    • Card inserted onto expansion slot on mother board

    • Manages all devices on SCSI bus

    • Supports internal and external SCSI devices

    • Forms a single daisy chain with devices

  • Device drivers

    • Enable OS to communicate with a host adapter

The scsi subsystem1
The SCSI Subsystem

  • Every SCSI bus subsystem requires a SCSI controller and unique SCSI IDs assigned to each device, including the host adapter

Issues to consider when selecting a host adapter
Issues to Consider when Selecting a Host Adapter

  • BIOS

  • Expansion slot

  • Bus mastering

Scsi device drivers
SCSI Device Drivers

  • Enable OS to communicate with a host adapter

  • Two popular driver types

    • ASPI (Advanced SCSI Programming Interface) – more popular of the two

    • CAM (Common Access Method)

Variations in scsi
Variations in SCSI

  • Bus width

  • Signaling methods used on SCSI cables

  • Connectors used with SCSI cables

  • Termination

  • SCSI-1, SCSI-2, and SCSI-3

Bus width
Bus Width

  • Narrow SCSI (8 bits)

  • Wide SCSI (16 bits)

Signaling methods used on scsi cables
Signaling Methods Usedon SCSI Cables

  • Single-ended (SE)

    • Send a signal on pair of twisted wires; one carries voltage, the other is a ground

  • Differential

    • Send a signal on pair of twisted wires; both carry voltage, and the signal is calculated to be the difference between the two voltages

    • High Voltage Differential (HVD)

    • Low Voltage Differential (LVD)


  • Prevents echo effect from electrical noise and reflected data at end of SCSI daisy chain

  • Single-ended SCSI cables use:

    • Passive terminators

    • Active terminators

    • Forced perfect terminators (FPTs)

  • Differential cables use either HVD or LVD terminators

Scsi 1 scsi 2 and scsi 3

SCSI-1 = Regular SCSI

8-bit data bus

7 possible devices

SCSI-2 = Fast SCSI

Common command set

16-bit data bus

15 possible devices

Mandatory parity checking

SCSI-3 = Ultra SCSI

Supports both parallel and serial data transmission

Supports FireWire connections

Data transfer of 320 MB/sec

SCSI-1, SCSI-2, and SCSI-3

Comparing ide and scsi
Comparing IDE and SCSI

  • IDE supports only four internal devices; SCSI supports both internal and external and allows you to add more

  • SCSI devices are generally of higher quality than IDE devices and more expensive

  • IDE devices require separate IRQ for each device; SCSI requires only one for entire chain


Comparing ide and scsi1
Comparing IDE and SCSI

  • Both are generally backward-compatible

  • SCSI hard drive with supporting host adapter and cable costs more than an IDE hard drive

  • SCSI subsystem provides faster data transfer than an IDE drive

  • SCSI generally provides better performance than IDE; often used on high-demand servers


Comparing ide and scsi2
Comparing IDE and SCSI

  • Good host SCSI host adapter allows connection of other SCSI devices to it

  • With SCSI, two or more devices can operate simultaneously; with IDE, only one IDE drive on an IDE channel can be busy at a time

Scsi installation
SCSI Installation

  • Set jumpers or switches on host adapter and install it on motherboard

  • Assign SCSI ID with jumpers or switches

  • Attach cabling to host adapter and each device

  • Verify termination at both ends of daisy chain

  • Power up one device at a time

  • Install drivers and software

Installing a host adapter card with windows 9x 2000 xp
Installing a Host Adapter Card with Windows 9x/2000/XP

  • Install card in expansion slot

  • Change/verify default settings using setup program

  • Verify settings under Advanced Configuration Options

  • Windows supports host adapter; it loads device drivers automatically and installs host adapter

  • Use Device Manager to verify correct installation of host adapter

  • Install external SCSI device

Setting device ids during installation
Setting Device IDs During Installation

  • With SCAM-compliance, SCSI IDs are assigned automatically

  • Without SCAM compliance, set device SCSI IDs manually

  • Each ID must be unique; no two devices on same SCSI channel can have same ID number

  • For narrow SCSI, use IDs 0-7

  • For wide SCSI, use IDs 0-15

  • Host adapter usually has ID 7

Setting device ids during installation1
Setting Device IDs During Installation

  • To set IDs for external devices, use either push-button selector or rotary selector

  • To set IDs for internal device, use set of jumpers on the device

Installing a scsi hard drive
Installing a SCSI Hard Drive

  • Make sure host adapter and cables are compatible with SCSI drive

  • Configure SCSI host adapter and SCSI hard drive so they can communicate with each other as follows:

    • Set SCSI IDs

    • Disable/enable disk drive and hard drive controllers

    • Check terminating resistors

    • Run CMOS setup for a SCSI system

    • Load SCSI device drivers

Troubleshooting scsi devices
Troubleshooting SCSI Devices

  • General tips

  • Installation problems

  • Booting problems

General troubleshooting tips
General Troubleshooting Tips

  • Keep notes and read documentation

  • Add one SCSI device at a time

  • Use good quality components

  • Limit cable length

Problems installing scsi devices
Problems Installing SCSI Devices

  • Turn on external devices first, then computer

  • Check connections

  • Verify correct termination (common cause of problems)

  • Verify most updated BIOS

  • Install PCI host adapter in PCI bus that supports bus mastering

Booting problems
Booting Problems

  • Confirm BIOS setup drive configuration is set to “No Drives Installed”

  • Verify that SCSI drive is partitioned, has a primary partition, and boot partition is set as active

  • Back up SCSI hard drive and do low-level format on it

Chapter summary
Chapter Summary

  • How SCSI technology works to provide better performance and greater expansion capabilities for many internal and external devices

  • Advantages and disadvantages of SCSI