Transcript A+ Guide to Managing and Maintaining Your PC, 5e

A+ Guide to Managing
and Maintaining Your PC
Fifth Edition
Chapter 22
All About SCSI
SCSI Basics
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
Standard for communication between
subsystem of peripheral devices and system
bus
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Bus can contain/be used by up to 7 or 15 devices
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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
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Mostly used in corporate settings
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The SCSI Subsystem
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Enables two devices on SCSI bus to pass data
between them without going through CPU
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Requires SCSI controller and unique SCSI IDs
assigned to each device, including host adapter
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Components

Host adapters
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Device drivers
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SCSI Host Adapter
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IDE versus SCSI Bus
Communication
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Host Adapters
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Manage all devices on SCSI bus
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Support internal and external SCSI devices
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Form a single daisy chain with devices
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Bus Mastering
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Bus master attached to PCI bus can access
memory and other devices without accessing
the CPU
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SCSI host adapter does not require a DMA
channel
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SCAM-Compliant
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Can assign SCSI IDs dynamically at startup
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Two levels
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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
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SCSI Device Drivers
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Enable OS to communicate with a host adapter
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Two popular types
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ASPI (Advanced SCSI Programming Interface) –
more popular of the two
CAM (Common Access Method)
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Variations in SCSI
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Bus width
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Signaling methods used on SCSI cables
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Connectors used with SCSI cables
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Termination
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SCSI-1, SCSI-2, and SCSI-3
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Bus Width
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Narrow SCSI
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8-bit data path
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Uses a cable with a 50-pin connector (A cable)
Wide SCSI
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16-bit data path
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Uses a cable with a 68-pin connector (P cable)
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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

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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
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Signaling Methods Used on
SCSI Cables (continued)
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Connectors Used with SCSI
Cables
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Not affected by signaling method used
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Types vary in shape and pin density; can be
connected with adapters
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Two main types:
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50-pin (A cable)
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68-pin (P cable)
Last connector on last device must be filled
with a terminator
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SCSI Connectors
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Multiple Connectors on the
SCSI Bus
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Termination
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Prevents echo effect from electrical noise and
reflected data at end of SCSI daisy chain
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Required at each end of a SCSI chain
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Ways to Terminate the End of a
SCSI Chain
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Switch setting on host adapter that activates or
deactivates a terminating resistor on the card
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Single SCSI connection or two connections
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Resistor physically mounted on the device
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Internal terminators that turn on or off with a
jumper setting on the device
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Software-controlled termination
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External SCSI Terminator
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Major Versions of SCSI
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SCSI-1 (Regular SCSI)
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No common command set
8-bit data bus
7 possible devices
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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Major Versions of SCSI
(continued)
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SCSI-3 (Ultra SCSI)
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Supports both parallel and serial data transmission,
and FireWire connections
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Data transfer of 320 MB/sec (Ultra 320 SCSI)
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Support up to 32 devices
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Includes SPI standard
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Comparing IDE and SCSI
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IDE
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May give better performance when using a single
disk drive with an OS like Windows 98 or
Windows Me
SCSI
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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
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Considerations When Choosing
Between IDE and SCSI Drives
IDE
SCSI
IDE supports only four internal Supports internal and external
devices
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
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More SCSI Considerations
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Drives perform better in RAID array
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Generally provides better performance
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Often used on high-demand servers
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Installing a SCSI Hard Drive
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Host adapter and cables must be compatible
with SCSI drive
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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
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SCSI Hard Drives Connectors
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Hands-on Project: Install a SCSI
Device
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pp. 1021
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