SCSI has a long progression. That is why there are many kind of SCSI in the present day. They are different in data width (8 or 16 bits) and working frequency (from 5 MHz to 80 MHz). With 8-bit data width, the parallel SCSI bus can be connected to 8 devices (used a 50 pin connector); with 16 bits, the SCSI bus can be connected to 16 devices.