Chapter 8
All About SCSI
You Will Learn…
• About basics of SCSI (Small Computer System
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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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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:

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
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 Used
on SCSI Cables
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Single-ended (SE)

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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
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SCSI-1 = Regular SCSI
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8-bit data bus
7 possible devices
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SCSI-3 = Ultra SCSI

SCSI-2 = Fast SCSI
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Common command set
16-bit data bus
15 possible devices
Mandatory parity
checking
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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
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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
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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 Installation
1.Set jumpers or switches on host adapter and
install it on motherboard
2.Assign SCSI ID with jumpers or switches
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 with
Windows 9x/2000/XP
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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
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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
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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.
2.
3.
4.
5.
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
• 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
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•
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
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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