Wednesday, October 01, 2008

Differential signaling

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.

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