The Free On-line Dictionary of Computing (30 December 2018):
A version of the SCSI command
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-disks. SCSI-2
addresses that problem. allowing scanners, hard disk
drives, CD-ROM drives, tapes and many other devices to be
Normal SCSI-2 equipment (not wide or differential) can be
connected to a SCSI-1 bus and vice versa.