The original design of the SCSI standard allowed two different types of buses, the single ended and differential. Single ended SCSI uses a single ended electrical signal (also refered to as unbalanced), this means that a single wire is used as a common ground return path for all signal lines. As of in the differential SCSI (refered to as balanced) each signal line has it's own return path and is also isolated from the reference ground. The cable uses twisted pair wiring, this makes it less prone to electrical interference and to picking up noise. This means that differential SCSI needs more wires and more electronics, and consequently also means higher prices, so as you guest most users systems use single ended SCSI because their cheaper and work good in most applications, like small systems and where electrical noise is not a problem.
Since differential SCSI is less prone to electical interference the cable lengths are greatly improved. On single ended SCSI, the cable length is limited to 6 meters (around 20 feet) and with differential SCSI the cable length can span up to 25 meters (about 82 feet). The shortest cable must not be less than 30 centimeters (12 inches) between each device. For example, if you have 3 device connected to the host adapter, the minimum lenght of the cable should be 1 meter (3 feet).
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