The standards of Small Computer System Interface has been developed in decades. SCSI-1 is the original and Ultra-320 is the currently used standard of Small Computer System Interface. Some considers SCA ( Single Connector Attachment ) as standard, but that is not true.
In general, SCSI devices are backward compatible , which means it is possible to use an ultra-3 hard drive to an Ultra-2 host bus adapter. Each SCSI device has a unique ID number. In the past, Small Computer System Interface was widespread on all types of computers. For high-performance workstations, servers and high-end peripherals that still applies today. Desktop computers and notebooks predominantly uses the slower ATA , Serial ATA interface and USB.
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The original Small Computer System Interface standards specified the physical properties of the buses and the electrical signaling as well as an instruction set that defined the different commands that could perform the Small Computer System Interface devices. This command set is also independent of the SCSI bus very useful because it is mature and is familiar to a large number of users and developers. iSCSI can serve as cost effective solution too.
Today, for example, the same cable is used for for Serial ATA (SATA) for Serial Attached Small Computer System Interface (SAS). in 1979 with the name SASI. In 1986 Small Computer System Interface became a standard specification of ANSI standard. Since then Small Computer System Interface has become the industry standard that could be used in almost any computer system.