   thumb|single ended parallel scsi icon/logo. small computer system interface (scsi,  ) is a set of standards for physically connecting and transferring data between computers and peripheral devices. the scsi standards define commands, protocols and electrical and optical interfaces. scsi is most commonly used for hard disks and tape drives, but it can connect a wide range of other devices, including scanners and cd drives, although not all controllers can handle all devices. the scsi standard defines command sets for specific peripheral device types; the presence of "unknown" as one of these types means that in theory it can be used as an interface to almost any device, but the standard is highly pragmatic and addressed toward commercial requirements. scsi is an intelligent, peripheral, buffered, peer to peer interface. it hides the complexity of physical format. every device attaches to the scsi bus in a similar manner. up to 8 or 16 devices can be attached to a single bus. there can be any number of hosts and peripheral devices but there should be at least one host. scsi uses handshake signals between devices, scsi-1, scsi-2 have the option of parity error checking. starting with scsi-u160 (part of scsi-3) all commands and data are error checked by a crc32 checksum. the scsi protocol defines communication from host to host, host to a peripheral device, peripheral device to a peripheral device. however most peripheral devices are exclusively scsi targets, incapable of acting as scsi initiatorsâunable to initiate scsi transactions themselves. therefore peripheral-to-peripheral communications are uncommon, but possible in most scsi applications. the symbios logic 53c810 chip is an example of a pci host interface that can act as a scsi target. ==history== scsi was derived from "sasi", the "shugart associates system interface", developed c. 1978 and publicly disclosed in 1981.ansi draft sasi standard, rev d, february 17, 1982, pg. ii states, "9/15/81 first presentation to ansi committee x3t9-3 (2 weeks following announcement in electronic design)." a sasi controller provided a bridge between a hard disk drive's low-level interface and a host computer, which needed to read blocks of data. sasi controller boards were typically the size of a hard disk drive and were usually physically mounted to the drive's chassis. sasi, which was used in mini- and early microcomputers, defined the in
Macintosh_internals SCSI