Chapter 22

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 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, depending on SCSI standard

Bus controller can be either:• An expansion card (host adapter)• Embedded on motherboard

The SCSI Subsystem

Host adapter• Gateway from the SCSI bus to the system bus

• 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

Two devices on SCSI bus can pass data between them without going through CPU; convenient for back up

The SCSI Subsystem

IDE vs. SCSI Bus Communication

The SCSI Subsystem

SCSI drive has its controller mounted inside the drive housing and can have a variable number of sectors per track

Each end of the SCSI chain has a terminating resistor Each device on the bus is assigned a SCSI ID Two important components

• Host adapters

• Device drivers

Host Adapters

Issues to consider when selecting a host adapter• BIOS

• Expansion slot

• Bus mastering

BIOS

Look for a host adapter that is PnP compatible and has a configuration utility built into its ROM

Check for software that configures termination and assigns system resources

Look for a host adapter whose BIOS can configure SCSI devices using the bus controlled by the adapter

See how many devices the BIOS supports (up to 15 is best)

Expansion Slot

Host adapter must fit the expansion slot you plan to use

Choose 32-bit PCI bus for faster data transfer rate (Pentium motherboard)

Bus Mastering

Choose a host adapter that supports bus mastering, if your system bus supports it

SCAM-Compliant

SCAM = SCSI Configuration AutoMatically Method by which SCSI devices and the host

adapter can be PnP compliant Two levels (Level 1 and Level2)

Vendors for SCSI Host Adapters

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

Standards Types of cabling Bus widths

Bus Width

Narrow SCSI (8 bits) Wide SCSI (16 bits)

SCSI Cables Signaling Methods

Single-Ended Signaling

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

Less expensive, but maximum cable length cannot be as long because data integrity is not as great

Differential Signaling

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

Better signal accuracy Greater reliability and reduction in chance of

data errors High Voltage Differential (HVD) Low Voltage Differential (LVD)

SCSI Cables

Signaling Symbols

SCSI Cable Connectors

SCSI Cable Connectors

Termination

Provides an echo effect from electrical noise and reflected data at the end of the SCSI daisy chain

Ways to Terminate a SCSI Chain

Host adapter can have a switch setting that activates or deactivates a terminating resistor on the card

Device can have either a single SCSI connection or two connections (external terminator)

Device at end of the chain can be terminated by a resistor physically mounted on that device

Internal terminator Controlled by software

Types of Terminators

Passive terminators Active terminators Forced perfect terminators (FPTs)

Major Versions of SCSI

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

• SPI (SCSI Parallel Interface) standards

SCSI Standards

SCSI Cable Specifications

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

continued…

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

continued…

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

Basic Steps for SCSI Installation

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

2. Use jumpers or switches to assign SCSI ID

3. Attach cabling to host adapter and each device

4. Verify termination at both ends of daisy chain

5. Power up one device at a time

6. Install drivers and software

Installing a Host Adapter Card

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

Installing a Host Adapter Card

Installing a Host Adapter Card

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 Installation

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

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

Setting External Device IDs

Setting Internal Device IDs

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:1. Set SCSI IDs2. Disable/enable disk drive and hard drive controllers3. Check terminating resistors4. Run CMOS setup for a SCSI system5. Load SCSI device drivers

General Troubleshooting Tips

Keep notes and read documentation Add one SCSI device at a time Use good quality components Limit cable length

Troubleshooting Installation Problems

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

Troubleshooting 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

How SCSI technology works to provide better performance and greater expansion capabilities for many internal and external devices (hard drives, CD-ROM drives, DVD drives, and scanners)

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

Advantages and disadvantages of SCSI