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The manner in which data is transmitted across a SCSI bus is defined by the method of signaling used. There are three types of SCSI signaling that can be used: High-voltage differential (HVD), Low-voltage differential (LVD) and Single Ended (SE).
HVD has been around since the earliest SCSI specifications (SCSI standards were discussed in SCSI Basics Part 1) and found popularity primarily due to the fact that HVD offers significant signal integrity, which allows for the utilizaton of longer cables without data loss or corruption. To give an example, Ultra2 SCSI using HVD can use cables up to 25 meters in length before the signal begins to corrupt, whereas Ultra2 SCSI using LVD can only use 12-meter cables.
However, for a couple of reasons, the ability to accommodate longer cables was not enough to make HVD the signaling method of choice. First, HVD uses two wires for each signal to increase signal integrity, making HVD expensive to implement and at the same time increasing its power requirements. The second reason is that in a practical situation, running a SCSI cable 25 meters is rarely necessary.