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• 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• 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 Subsystem• Every SCSI bus subsystem requires a SCSI

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

The SCSI Subsystem

IDE vs. SCSI Bus Communication

Issues to Consider when Selecting a Host Adapter

• BIOS

• Expansion slot

• Bus mastering

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• 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 bits)

• Wide SCSI (16 bits)

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)

Signaling Methods on SCSI Cables

SCSI Cables

Signaling Methods on SCSI Cables

Connectors for SCSI Cables

Connectors for SCSI Cables

Termination• 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

Termination

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

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

SCSI Installation1.Set jumpers or switches on host adapter and

install it on motherboard2.Assign SCSI ID with jumpers or switches3.Attach cabling to host adapter and each device4.Verify termination at both ends of daisy chain5.Power up one device at a time6.Install drivers and software

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

Installing a Host Adapter Card with Windows 9x/2000/XP

Installing a Host Adapter Card with Windows 9x/2000/XP

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 device, 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

Troubleshooting SCSI Devices• General tips

• Installation problems

• Booting problems

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• 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• 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

• Advantages and disadvantages of SCSI