SCSI Types

There are really only three basic specifications of SCSI:
•SCSI-1: The original specification developed in 1986
• SCSI-2: An update that became an official standard in 1994, a key component of SCSI-2 was the inclusion of the Common Command Set (CCS) -- the 18 commands considered an absolute necessity for support of any SCSI device. You also had the option to double the clock speed from 5 MHz (million cycles per second) to 10 MHz (Fast SCSI), double the bus width from 8 bits to 16 bits and increase the number of devices to 15 (Wide SCSI), or do both (Fast/Wide SCSI). Finally, SCSI-2 added command queuing, which means that an SCSI-2 device can store a series of commands from the host computer and determine which ones should be given priority.
• SCSI-3: Quickly on the heels of SCSI-2 came SCSI-3, debuting in 1995. The interesting thing about SCSI-3 is that a series of smaller standards have been built within its overall scope. Because of this continually evolving series, SCSI-3 is not considered to be a completely approved standard. Instead, some of the specifications developed within it have been officially adopted. These standards are based on variations of the SCSI Parallel Interface (SPI), which is the way that SCSI devices communicate with each other. Most SCSI-3 specifications begin with the term "Ultra" (Ultra for SPI variations, Ultra2 for SPI-2 variations and Ultra3 for SPI-3 variations). The Fast and Wide designations work just like their SCSI-2 counterparts, with the Fast designation meaning that the clock speed is double that of the base version, and the Wide designation meaning that the bus width is double that of the base. The chart below shows a comparison of the many SCSI variations: Name Specification # of Devices Bus Width Bus Speed MBpsAsynchronousSCSI SCSI-1 8 8 bits 5 MHz 4 MBpsSynchronousSCSI SCSI-1 8 8 bits 5 MHz 5 MBpsWideSCSI SCSI-2 16 16 bits 5 MHz 10 MBpsFastSCSI SCSI-2 8 8 bits 10 MHz 10 MBpsFast/WideSCSI SCSI-2 16 16 bits 10 MHz 20 MBpsUltraSCSI SCSI-3SPI 8 8 bits 20 MHz 20 MBpsUltra/WideSCSI SCSI-3SPI 8 16 bits 20 MHz 40 MBpsUltra2SCSI SCSI-3SPI-2 8 8 bits 40 MHz 40 MBpsUltra2/WideSCSI SCSI-3SPI-2 16 16 bits 40 MHz 80 MBpsUltra3SCSI SCSI-3SPI-3 16 16 bits 40 MHz 160 MBps