High-voltage differential signalling
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High-voltage differential (HVD) signalling is an electrical signalling system using high-voltage signals, as opposed to low-voltage differential signalling (see LVDS). In computer electronics, "high voltage" normally means 5 volts or more.
Differential circuitry normally allows longer cables than single-ended signalling. This is because the signal on the wires is received not as the difference between a signal wire and a ground wire, but as the difference between two wires not related to ground (hence the term "differential"). Since any interference to the signals coming from external sources is likely to influence the two wires by an equal amount, leading to a 'common mode' signal which is removed by the receiver, the maximum cable length is increased compared with single-ended circuitry.
SCSI-1 variations included a high voltage differential (HVD) implementation whose maximum cable length was many times that of the single-ended version. SCSI equipment for example allows a maximum total cable length of 25 meters using HVD, while single-ended SCSI allows a maximum cable length of 1.5 to 6 meters, depending on bus speed. Note that LVD versions of SCSI allow less than 25 m cable length not because of the lower voltage, but because these SCSI standards allow much higher speeds than the older HVD SCSI.