Source : Free On-Line Dictionary of Computing

SCSI-2
     
         A version of the {SCSI} command
        specification.
     
        SCSI-2 shares the original SCSI's {asynchronous} and
        {synchronous} modes and adds a "{Fast SCSI}" mode (<10MB/s)
        and "{Wide SCSI}" (16 bit, <20MB/s or rarely 32 bit).
     
        Another major enhancement was the definition of command sets
        for different device classes.  SCSI-1 was rather minimalistic
        in this respect which led to various incompatibilities
        especially for devices other than {hard-disk}s.  SCSI-2
        addresses that problem. allowing {scanner}s, {hard disk
        drive}s, {CD-ROM} drives, tapes and many other devices to be
        connected.
     
        Normal SCSI-2 equipment (not wide or {differential}) can be
        connected to a SCSI-1 bus and vice versa.
     
        (1995-04-19)