SCSI Interface

SCSI (Small Computer Systems Interface)

SCSI (pronounced scuzzy) is the acronym for Small Computer System Interface, a high performance peripheral interface that can independently distribute data among peripherals attached to the PC. Unlike ATA, SCSI incorporates those instructions needed for communication with the host PC. Freeing the host computer from this job makes it more efficient in performing its user-oriented activities. SCSI is a specification for a peripheral bus and command set defined in an ANSI standard X3.131-1986. SCSI drives are usually more suitable for high-end computer systems, which require maximum possible performance. SCSI provides for higher data transfer rates and less CPU load than ATA but has higher cost and complexity in the setup. Also, SCSI supports more devices than ATA. Another important advantage of SCSI is that most SCSI products are backward-compatible. A faster, newer drive can still work with the older and slower controller, but will, of course, lose in performance.

The history of SCSI goes back to 1981, when Shugart Associates (presently, Seagate Technology Corporation) joined NCR Corporation in their efforts to develop an intelligent interface for disk drives. This new interface was called Shugart Associates Systems Interface or SASI interface. In 1982, SCSI was born on base of SASI. In 1986, ANSI accepted SCSI as an ANSI standard.

There are different flavor of SCSI, different in width (8-bit vs. 16-bit) and speed (Standard vs. Fast). Fast SCSI doubles the data transfer rate over the same 50-pin cable by using synchronous (vs. asynchronous) data transfer.

Wide SCSI uses "wider" 68-pin cable to allow 16-bit data stream vs. 8-bit for 50-pin connector. A combination of both wide and fast technologies provides for the greater then 80 MB/s data transfer rates.
The next two figures show SCSI 50-pin (8-bit) A-cable and 68-pin (16-bit) P-cable female connectors:

-----------------------------------
! 1 ooooooooooooooooooooooooo 49 !
! 2 ooooooooooooooooooooooooo 50 !
!----------------------------------
--------------------------------------------
! 1 ooooooooooooooooooooooooooooooooo 67 !
! 2 ooooooooooooooooooooooooooooooooo 68 !
!------------------------------------------!



The first and the last SCSI devices, connected in the Daisy chain, should be properly terminated. The first device is the SCSI controller itself. The devices in between should not be terminated.
reference from : http://www.usbyte.com/common/SCSI_Interface.htm