Advantages of the Small Computer System Interface (SCSI) (103435)

SUMMARY

The following are advantages of the small computer system interface (SCSI):

1. Unlike other interfaces, when you interface with different device types using SCSI, the interfacing is done through the same cable. In a non-SCSI environment, devices such as a proprietary tape controller, disk controller, and so on, must be used to connect their respective devices to the system bus.
2. SCSI peripheral devices of the same type have similar characteristics (this makes it easy to replace old devices with new ones).
3. SCSI peripheral devices are intelligent and independent: a controller is built onto each SCSI device. This allows the computer to do other work.
4. SCSI I/O is independent of the system bus. This allows peripheral devices to work with different computer types, which preserves a company's hardware investment.
5. SCSI is fast (10 megabytes (MB)/second on 8 bit bus, 20 MB/second on 16 bit bus).
6. Multi-threaded operating systems, such as Windows NT, can take full advantage of the multi-tasking capabilities of the SCSI bus.

For example, when a Windows NT thread requests to read a logical block on SCSI disk 1, and at the same time, a second thread requests to write some data to SCSI disk 2, the following may occur:

• The SCSI host adapter will process the first request made by Windows NT executive by arbitrating the SCSI bus and making a connection to disk 1.
• After the connection is made, disk 1 will disconnect and give up the bus (bus free) so that other requests can be made by the host.
• The first thread will stop executing and will wait while the slow I/O device completes a data transfer.
• As the seek is carried out on disk 1, the second thread request will be processed in the same manner as the first, because Windows NT can issue a "context switch" to allow for a thread of execution while another is still being completed.
• Because the bus is free at this time, the host (initiator) will be able to make a connection with disk 2. Disk 2 will then disconnect and perform a write of some data to a logical location on the disk. At the same time, disk 1 may still be seeking the block to read. The two devices are therefore performing a task ( read, write) simultaneously.