Differential Signaling Mechanism

　・In Japanese
Premise knowledge
　・Serial communication
　・Unidirectional communication
　・comparator
　・H bridge circuit

■What is Differential Signaling?

Differential signaling is a serial communication method that uses two signal lines to increase noise resistance, resulting in high-speed communication. This method can coexist with unidirectional communication, and even if it is unidirectional communication, it uses two signal lines. The circuit diagram and signal behavior are shown below. Output P is positive and the output is high, and N is negative and the output is low.

Differential signaling is resistant to noise for two reasons. One is that when noise is added to the transmission path, the noise is applied equally to both the P side and the N side, and the difference remains the same.



The second reason is that the forward and return currents of the transmission line are in opposite directions, in which case the magnetic field is canceled out and noise is less likely to occur. The cables used include twisted pair cables, which are twisted cables to help cancel out magnetic fields.

■Differential signaling working mechanism

The receiver uses a comparator and the driver uses an H-bridge circuit. This shows the current flow when switches A and D are turned on. At this time, the voltage level is High for P and Low for N.



Next, when switches B and C are turned on, the current flow is as follows. The voltage level is low for P and high for N. In this way, by switching the switch (realized with a transistor), the voltage level between PN is changed.



The above is the mechanism of Differential signaling. In contrast to differential signaling, a method of communicating using one line instead of two lines is called single-end signal transmission.

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