Chapter22

A+ Guide to Managing and Maintaining Your PCFifth Edition

Chapter 22

All About SCSI

2 A+ Guide to Managing and Maintaining Your PC, Fifth Edition

You Will Learn…

About basics of SCSI technology and components

How SCSI hard drives compare to IDE drives

How to install a SCSI device

Troubleshooting tips for SCSI


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 Bus requires controller embedded on motherboard

or host adapter inserted into an expansion slot

Faster, more expensive, more difficult to install than similar IDE devices Mostly used in corporate settings


The SCSI Subsystem

Enables two devices on SCSI bus to pass data between them without going through CPU

Requires SCSI controller and unique SCSI IDs assigned to each device, including host adapter

Components

Host adapters

Device drivers


SCSI Host Adapter


IDE versus SCSI Bus Communication


Host Adapters

Manage all devices on SCSI bus

Support internal and external SCSI devices

Form a single daisy chain with devices


Considerations When Selecting a Host Adapter BIOS

Expansion slot

Bus mastering

SCAM-compliant


BIOS Considerations Plug and Play compatibility

Built-in configuration utility

Software that configures termination automatically and assigns system resources

Ability to configure SCSI devices using the bus controlled by the adapter

Support for up to 15 peripherals


Expansion Slot Considerations

Host adapter must fit expansion slot(eg, 16-bit ISA, PCI)


Bus Mastering

Bus master attached to PCI bus can access memory and other devices without accessing the CPU

SCSI host adapter does not require a DMA channel


SCAM-Compliant Can assign SCSI IDs dynamically at startup

Two levels

Level 1 requires that the devices – but not the host adapter – be assigned an ID at startup by software

Level 2 requires that the host adapter – as well as the devices – be assigned an ID at startup by software


SCSI Device Drivers

Enable OS to communicate with a host adapter

Two popular types

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

CAM (Common Access Method)


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

Narrow SCSI

8-bit data path

Uses a cable with a 50-pin connector (A cable)

Wide SCSI

16-bit data path

Uses a cable with a 68-pin connector (P cable)


Signaling Methods Used on SCSI Cables

Single-ended (SE) Differential

Send signal on pair of twisted wires – one carries voltage, one is a ground

Less expensive than differential

Shorter maximum cable length; data integrity is not as great

Send signal on pair of twisted wires – both carry voltage; signal calculated to be the difference between the two

Better signal accuracy and greater reliability

HVD and LVD


Signaling Methods Used on SCSI Cables (continued)


SCSI Cables


Signaling Symbols


Connectors Used with SCSI Cables Not affected by signaling method used Types vary in shape and pin density; can be

connected with adapters Two main types:

50-pin (A cable) 68-pin (P cable)

Last connector on last device must be filled with a terminator


SCSI Connectors


Multiple Connectors on the SCSI Bus


Termination

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

Required at each end of a SCSI chain


Ways to Terminate the End of a SCSI Chain Switch setting on host adapter that activates or

deactivates a terminating resistor on the card

Single SCSI connection or two connections

Resistor physically mounted on the device

Internal terminators that turn on or off with a jumper setting on the device

Software-controlled termination


External SCSI Terminator


Terminators Types

Passive terminators Active terminators Forced perfect terminators (FPTs)

Single-ended SCSI cables use passive terminators, active terminators, and FPTs

Differential cables use either HVD or LVD terminators


Major Versions of SCSI SCSI-1 (Regular SCSI)

No common command set 8-bit data bus 7 possible devices

SCSI-2 (Fast SCSI) Common command set 16-bit data bus 15 possible devices Mandatory parity checking


Major Versions of SCSI (continued) SCSI-3 (Ultra SCSI)

Supports both parallel and serial data transmission

Supports FireWire connections

Data transfer of 320 MB/sec

Includes SPI standard


SCIS Standards


SCIS Standards (continued)


SCIS Standard Cable Specs


Comparing IDE and SCSI

IDE May give better performance when using a single

disk drive with an OS like Windows 98 or Windows Me

SCSI Increases performance where heavy load is placed

on system and its components Some OSs include increased support for SCSI

features and take better advantage of them More expensive; better performance


Considerations When Choosing Between IDE and SCSI Drives

IDE SCSI

IDE supports only four internal devices

Supports internal and external devices; allows additional ones

Separate IRQ required for each device

Only one IRQ required for entire chain

Generally backward-compatible Generally backward-compatible

Only one IDE drive on an IDE channel can be busy at a time

Two or more devices can operate simultaneously


More SCSI Considerations

Drives perform better in RAID array

Devices are generally of higher quality and more expensive

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


More SCSI Considerations (continued) Subsystem provides faster data transfer

Generally provides better performance

Often used on high-demand servers

A good SCSI host adapter allows connection of other SCSI devices to it


Basic Steps for Installing SCSI Devices

1. Set jumpers or switches on host adapter; install it on motherboard; install host adapter drivers

2. Assign SCSI ID with jumpers or switches

3. Attach cabling to host adapter and each device


Basic Steps for Installing SCSI Devices (continued)

4. Verify termination at both ends of SCSI chain

5. Power up one device at a time

6. Install drivers and software needed to interface between SCSI subsystem and OS


Installing a Host Adapter Card

Install card in expansion slot

Change/verify default settings (setup program)

Verify settings (Advanced Configuration Options)


Installing a Host Adapter Card (continued) Windows supports host adapter

Loads device drivers automatically and installs host adapter

Verify correct installation of host adapter (Device Manager)

Install external SCSI device


Installing an External SCSI Device Install software to run device (includes driver)

Plug SCSI cable into host adapter port

Plug other end of cable into device

Set SCSI ID and install terminator on device

Connect device’s power cord to wall outlet; turn on device

Restart PC and test device


Setting Device IDs During Installation Assigned automatically with SCAM-compliant

SCSI subsystem

Set manually without SCAM compliance; each ID must be unique

For narrow SCSI, use IDs 0-7

For wide SCSI, use IDs 0-15

Host adapter usually has ID 7


Setting Device IDs During Installation (continued) For external device:

Push-button or rotary selector

For internal device:

Set of jumpers on the device


Setting IDs for External Devices


Setting IDs for Internal Devices


Installing a SCSI Hard Drive Host adapter and cables must be compatible

with SCSI drive

Configure SCSI host adapter and SCSI hard drive to communicate with each other:

1. Set SCSI IDs

2. Disable/enable disk drive and hard drive controllers

3. Check terminating resistors

4. Run CMOS setup for a SCSI system

5. Load SCSI device drivers


SCSI Hard Drives Connectors


General Tips for Troubleshooting SCSI Devices Keep notes and read documentation

Add one SCSI device at a time

Use good quality components

Limit cable length


Troubleshooting SCSI Installation Problems Turn on external devices first, then computer

Check all connections

Check termination (common cause of problems)

Verify most updated BIOS

Install PCI host adapter in PCI bus that supports bus mastering


Troubleshooting SCSI Boot 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


Summary

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

Advantages and disadvantages of SCSI

Installation and troubleshooting guidelines

Chapter22

Technology

scsi ide

scsi subsystem

major versions of scsi

scsi features

scsi cablesconnectors

regular scsi

cablewide scsi

ultra scsi