This article is from the storage FAQ part2, by Rodney D. Van Meter with numerous contributions by others.
SCSI (now commonly known as SCSI-I) was the original 1986 standard, X3.131-1986. It specified the electrical level and some of the mid-layer issues involving messages and packet structure, but (I believe, my memory's bad) didn't formalize the Common Command Set (CCS), that was done independently. It supported a maximum burst rate of 5 MB/sec. on an 8-bit bus. ADDITIONAL INFORMATION Consult the SCSI standards documents, and the manuals for the device you are working with for more information. The "SCSI 1" specification document is called SCSI Specification, ANSI X3T9.2/86-109. Also of interest is the Common Command Set specification document SCSI CCS Specification, ANSI X3T9.2/85-3 SCSI-II received final approval in early 1994, but has been a de facto standard for several years. The CCS was standardized for a variety of different types of peripherals. The max allowable transfer rate was raised to 10 MT/s (see below). A 16-bit bus (Wide SCSI) and 32-bit bus (double-wide SCSI) are specified (see below). SCSI-III is the latest effort, and involves more cleanly separating the functionality into layers; the command layer is defined independently from the physical layer. In addition to the traditional parallel cable, there are efforts going on to define physical layers for Fibre Channel and a more generic Serial SCSI. Thus, there will be no SCSI-IV; only the individual pieces will be updated as necessary.