*** info source: http://www.cs-electronics.com/68-pin-internal-scsi-cables/ ***

• SCSI-1
  The original standard that was derived from the Shugart Associates System Interface (SASI) and formally adopted in 1986 by ANSI. SCSI-1 features an 8-bit parallel bus (with parity), running asynchronously at 3.5 MB/s, or 5 MB/s in synchronous mode, and a maximum bus cable length of 6 meters (just under 20 feet-compared to the 18 inch (0.45 meter) limit of the ATA interface). A rarely seen variation on the original standard included a high-voltage differential (HVD) implementation whose maximum cable length was 25 meters.
   

• SCSI-2
  SCSI-2 was introduced in 1994 and gave rise to the Fast SCSI and Wide SCSI variants.  Fast SCSI doubled the maximum transfer rate to 10 MB/s and Wide SCSI doubled the bus width to 16 bits on top of that to reach a maximum transfer rate of 20 MB/s.  However, these improvements came at the cost of reducing the maximum cable length to three meters.  SCSI-2 also specified a 32-bit version of Wide SCSI, which used two 16-bit cables per bus.  The 32-bit implementation was largely ignored because it was expensive and unnecessary, and was officially retired in SCSI-3.
   

• SCSI-3
  Before Adaptec and later SCSITA codified the terminology, the first parallel SCSI devices that exceeded the SCSI-2 capabilities were simply designated SCSI-3. These devices, also known as Ultra SCSI or Fast-20 SCSI, were introduced in 1996. The bus speed doubled again to 20 MB/s for narrow (8 bit) systems and 40 MB/s for wide (16-bit). The maximum cable length stayed at 3 meters but single-ended Ultra SCSI developed an undeserved reputation for extreme sensitivity to cable length and condition (faulty cables, connectors or terminators were often to blame for instability problems).
  Unlike previous SCSI standards, SCSI-3 (Fast-20 speed) requires active termination.
   

• Ultra-2
  This standard was introduced in 1997 and featured a low-voltage differential (LVD) bus. For this reason ultra-2 is sometimes referred to as LVD SCSI. LVD's greater resistance to noise allowed a maximum bus cable length of 12 meters. At the same time, the data transfer rate was increased to 80 MB/s. Mixing earlier single-ended devices (SE) and Ultra-2 devices on the same bus is possible but connecting only a single SE device forces the whole bus to single-ended mode with all its limitations, including transfer speed. Ultra-2 SCSI actually had a relatively short lifespan, as it was soon superseded by Ultra-3 (Ultra-160) SCSI.
   

• Ultra-3
  Also known as Ultra-160 SCSI and introduced toward the end of 1999, this version was basically an improvement on the ultra-2 standard, in that the transfer rate was doubled once more to 160 MB/s by the use of double transition clocking. Ultra-160 SCSI offered new features like cyclic redundancy check (CRC), an error correcting process, and domain validation, a way to negotiate maximum performance for each device on the chain.
   

• Ultra-320
  This is the Ultra-160 standard with the data transfer rate doubled to 320 MB/s. The latest working draft for this standard is revision 10 and is dated May 6, 2002. Nearly all SCSI hard drives being manufactured at the end of 2003 were Ultra-320 devices.
   

• Ultra-640
  Ultra-640 (otherwise known as Fast-320) was promulgated as a standard (INCITS 367-2003 or SPI-5) in early 2003. It doubles the interface speed yet again, this time to 640 MB/s. Ultra-640 pushes the limits of LVD signaling; the speed limits cable lengths drastically, making it impractical for more than one or two devices. Because of this, manufacturers have skipped over Ultra640 and are developing for Serial Attached SCSI instead.

*** Info source: https://en.wikipedia.org/wiki/Parallel_SCSI#cite_note-1 ***