SCSI SCSI Basics Standard for communication between subsystem

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) Send signal on pair of twisted wires – one carries voltage, one is a ground l Less expensive than differential l Shorter maximum cable length; data integrity is not as great l Differential Send signal on pair of twisted wires – both carry voltage; signal calculated to be the difference between the two l Better signal accuracy and greater reliability l HVD and LVD l

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 – 5 MHz sync and async operations • SCSI-2 (Fast SCSI) – 50 pin cable High density. Fast SCSI(10 MHz) – 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 Fire. Wire 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; allows additional ones Separate IRQ required for each device Only one IRQ required for entire chain 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