The High voltage differential reference article from the English Wikipedia on 24-Apr-2004
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High voltage differential

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High voltage differential stands for an electrical signalling system using high voltage, as in opposite to low voltage differential signalling (see LVD). Some toggle switches control large amounts of voltage. Alternating currents can be stepped up by a transformer to a high voltage. Because of Ohm's law, electrical energy losses are dependent on current flow, not on energy flow. By using transformers, the voltage of the power can be stepped up or stepped down.

Computers

In the computer arena, "high voltage" normally stands for 5 volts or more. Such circuitry normally allows longer cable length than so called single ended versions. This is because the signal on the wires is not received as a delta of a signal wire to ground, but as the difference between pairs of wires not related to ground (hence the term "differential"). Since any distortion to the signals coming from external sources are likely to influence the pairs by an equal amount, the maximum cable length is increased when related to 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 25m cable length not because of the lower voltage, but because these SCSI standards allow much higher speeds than the older HVD SCSI.

Transmission

Efficient electricity transmission and distribution systems operate over long distances at very high voltages. The principles regarding frequency and power level for the first primitive RADAR units used high voltage, high frequency alternating currents.

See also