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An RS422 communication link is a four-wire link with balanced line drivers. In a balanced differential system, one signal is transmitted using two wires (A and B). The signal state is represented by the voltage across the two wires. Although a common signal ground connection is necessary, it is not used to determine the signal state at the receiver. This results in a high immunity against EMI (electromagnetic interference) and allows cable lengths of over 1000m, depending on the cable type and baud rate.

 

The EIA Standard RS422-A "Electrical characteristics of balanced voltage digital interface circuits" defines the characteristics of an RS422 interface.

 

Transmitter and receiver characteristics according to RS422-A are:

 

 

Permitted Common Mode Voltage Vcm (mean voltage of A and B terminals with reference to signal ground): -7V to +7V