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Differential signaling is a method of transmitting information over pairs of wires (as opposed to single-ended signalling, which transmits information over single wires).
Differential signaling reduces the noise on a connection by rejecting common-mode interference. Two wires (referred to here as A and B) are routed in parallel, and sometimes twisted together, so that they will receive the same interference. One wire carries the signal, and the other wire carries the inverse of the signal, so that the sum of the voltages on the two wires is always constant.
At the end of the connection, instead of reading a single signal, the receiving device reads the difference between the two signals. Since the receiver ignores the wires' voltages with respect to ground, small changes in ground potential between transmitter and receiver do not affect the receiver's ability to detect the signal. Also, the system is immune to most types of electrical interference, since any disturbance that lowers the voltage level on A will also lower it on B.
Some communications protocols that use differential signaling include RS-422, RS-485, and USB.
Differential vs. single-ended signaling
A differential signaling system is unlike the more common technique of single-ended signaling, in which the transmitter generates a single voltage that the receiver compares with a fixed reference voltage, both relative to a common ground connection shared by both ends.
The widely used RS-232 system is an example of single-ended signaling, which uses ±12 V to represent a signal, and anything less than ±3 V to represent the lack of a signal. The high voltage levels give the signals some immunity from noise, since few naturally occurring signals can create that sort of voltage. They also have the advantage of requiring only one wire per signal. However, they also have a serious disadvantage: they cannot run at high speeds. The effects of capacitance and inductance, which filter out high-frequency signals, limit the speed. Large voltage swings driving long cables also require significant power from the transmitting end. This problem can be reduced by using smaller voltages, but then the chance of mistaking random environmental noise for a signal becomes much more of a problem. Another difficulty is the electromagnetic interference that can be generated by a single-ended signaling system which attempts to operate at high speed.
Differential signaling uses the difference in voltage between two wires to signal information. This system has a lower susceptibility to noise, because distant noise sources tend to add the same amount of voltage (called common-mode noise) to both wires, so the difference between the voltages remains the same. Manufacturers can further reduce noise by twisting the two wires of a pair together (as in Cat-3 Ethernet cables), so that any noise induced in one half-twist tends to cancel the noise induced in the neighboring half-twist.
Other examples include differential ECL, PECL, LVPECL, RS-422 and RS-485. Low voltage differential signaling is currently the only scheme that combines low power dissipation with high speed.
=High-voltage differential signaling= High-voltage differential (HVD) signaling uses high-voltage signals, as opposed to low-voltage differential signaling (see LVDS). In computer electronics, "high voltage" normally means 5 volts or more.
Differential circuitry normally allows longer cables than single-ended signaling. 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.
See also
"Differential signaling"