Small Computer System Interface versions differ in the number of bit per transfer, speed and types of signal.
(a) number of bits
SCSI (normal) - 8 bits, 5MB/s
SCSI (wide) - 16-bits, 10MB/s
(b) speed (frequency)
Fast SCSI, Ultra, Ultra-2, Ultra-3 (Ultra-160) and Ultra-320
(c) Signal
Single-ended (SE) use a strong voltage for 1 and no voltage for 0. This method is not stable with higher speed and longer cable.
Differential Voltage uses 2 wires. No voltage on both wire represents 0. Opposite voltage applies to the wires to represent 1. HVD (Hign Voltage Differential) was the inital standard. LVD was now the more common type. Some LVD can operate in SE mode but results in slow speed.
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