SCSI-3

The most recent SCSI standard is SCSI-3. It differs from the old SCSI-2 spec in that it contains many more new features, technologies, and command sets. Instead of going with a single document that covered everything in SCSI-3, the decision was made to have SCSI-3 be a collection of standards that were related to each other. This would allow groups to independently work on the standards separately, and would allow popular ones to progress faster than others. Work started on this new collection of standards a year before SCSI-2 was released.

SCSI-3 covers many more technologies and standards compared to SCSI-2. Some would say, maybe too many. Since SCSI-3 really refers to so many technologies, many people get confused by the term. SCSI-3 hasn't been approved over-all, but the some elements of the standard has been in use already. It would probably take me awhile to get into the entire SCSI-3 spec, so I'll leave that as an exercise for the more curious readers out there. There's so much changing in the spec every month that any info I provide might be outdated rather quickly. However, I will get into some of the improvements made into the SCSI-3 interface, known as the SCSI-3 Parallel Interface (SPI). This interface has evolved into SPI-2, SPI-3, and SPI-4.

The original SPI included specs for the protocols used for parallel SCSI as well as Fast SCSI. Fast-20 was later added to the spec which defined a faster 20 MHz bus, increasing the transfer rate to 40 MB/s. SPI-2 added even more changes, which included Fast-40, Low Voltage Differential Signaling (LVD), multimode devices, new connectors, and more. Fast-40 again doubles the transfer rate to 80 MB/s on a wide channel. To replace the old SE and HVD signaling methods, LVD was developed and added to the spec. This method involves pretty much the same concept as HVD, except low voltages are used. This lowers cost and power consumption. LVD has pretty much replaced HVD in modern SCSI systems, and it's even required for the fastest SCSI buses. Ultra2 SCSI and Wide Ultra2 SCSI are the informal marketing terms for devices that conform to the SPI-2 standard.

The SPI-3 standards again doubled the transfer rate to 160 MB/s with Fast-80 data transfer. Lots of performance enhancements were added to this evolution of SPI, as well as removing some obsolete standards. The problem with this set of standards is that any device that uses any subset of the new features were called Ultra3 SCSI devices. That means that two devices can be both Ultra3 but still be incompatible with each other. To solve this, new marketing terms were coined to identify these new devices. The result is Ultra160 SCSI and Ultra160+ SCSI. SPI-4 is currently still in the works. It includes Fast-160 which yet again doubles the transfer rate to 320 MB/s. Despite still being in the development stage, devices are already using the new standard, Ultra320. Other than that, it's not really clear what else may be included in the new spec.