Small Computer System Interface

SCSI (formally 'Small Computer System Interface') was a popular interface standard for high-speed devices (disks, tape drives, scanners, etc). It was composed of a set of standards for the various layers: physical signalling, commands, etc.

Over time, a variety of options were developed for many of these; e.g. the original standard was 8-bit parallel using single-ended signalling, but later versions used 16-bit with differential pairs, at a variety of speeds (mostly synchronous, but one was asynchronous). Even later, a high-speed serial version was developed; and ultimately, one which worked over TCP/IP (usually over an Ethernet).