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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:


Signal State

Transmitter Differential Voltage VAB

Receiver Differential Voltage VAB

Mark (or logical 1)

-6V to -2V

-6V to -200mV

Space (or logical 0)

+2V to +6V

+200mV to 6V


-2V to +2V

-200mV to +200mV


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