Why is RS-422 interface interference insensitive?
Summary
In a balanced signal both wires carry the signal, with one wire the negative of the other. In the receiver both are subtracted, giving the double of the positive wire's signal. If both wires pick up a disturbance this will be cancelled by the subtraction.  



Twisted wires reduce the inductance because the field inverts with each half twist. An inductive disturbance induced in one half twist will therefore be cancelled by the next half twist.  

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