Data Transfer Modes

Well this is the section you've all been waiting for. What's with all this Ultra SCSI and Wide Ultra SCSI nonsense? These terms basically refer to the different transfer modes which determine the speed and width of the SCSI bus. The different SCSI standards have defined these transfer modes. I won't bother going into the modes defined in the SCSI-1 or SCSI-2 standards but I will mention them so that you know they exist. For convenience sake, I'll just put up a table of the different modes grouped by which SCSI standard they were defined in. The bus speed, width, and transfer speed are also given.

Transfer Modes Defined in SCSI-1
Name	Bus Width (Bits)	Bus Speed (MHz)	Transfer Speed (MB/s)
SCSI	8	5	5

Transfer Modes Defined in SCSI-2
Name	Bus Width (Bits)	Bus Speed (MHz)	Transfer Speed (MB/s)
Fast SCSI	8	10	10
Wide SCSI	16	5	10
Fast Wide SCSI	16	10	20

Transfer Modes Defined in SCSI-3
Name	Bus Width (Bits)	Bus Speed (MHz)	Transfer Speed (MB/s)
Ultra SCSI	8	20	20
Wide Ultra SCSI	16	20	40
Ultra2 SCSI	8	40	40
Wide Ultra2 SCSI	16	40	80
Ultra160 SCSI	16	40	160
Ultra160+ SCSI	16	40	160
Ultra320 SCSI	16	80	320

As you can see that there are many different flavors of SCSI, and the average person will most likely be confused. Not only do these modes differ in their bus width and speed, but also in the type of connectors used, the termination method, the signaling method, the number of devices supported and the max cable length. Basically any mode that has a bus width of 8-bits uses 50-pin cables, and any mode with a bus width of 16-bits uses 68-pin cables. Also, anything after Ultra2 SCSI uses LVD for it's signaling method. The older modes use SE and HVD. In addition to the increased speed, the newest modes also include some special features defined in the SPI-3 and SPI-4 spec to improve performance and reliability. You might be wondering what's the difference between Ultra160 and Ultra160+. Well, Ultra160+ includes two more key features from the SPI-3 Ultra3 SCSI spec. Ultra160 only has 3 of the 5 features in Ultra3, while Ultra160+ has all 5. The future of SCSI is still unclear since SCSI-3 hasn't even been finalized, but I'm sure there's a lot more in store.