Transcript - Pcpolytechnic

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
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
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
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
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)
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
External SCSI Terminator
Major Versions of SCSI
• SCSI-1 (Regular SCSI)
– 50 pin cable
– 8-bit data bus
– 7 possible devices
– 5MHz sync and async operations
• SCSI-2 (Fast SCSI)
– 50 pin cable High density. Fast SCSI(10MHz)
– 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
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
SCSI Hard Drives Connectors