Answer / vikram budhiraja

A process variable can be transmitted (as an analog signal) over a pair of wires either as a voltage or a current.

A current (milliamp) analog signal is preferred in control systems over voltage because the current in a loop cannot be different at the generating end and the receiving end regardless of the length of the wire carrying the signal.

On the other hand a voltage signal suffers a drop ( IR loss) depending on the length of the wires carrying the signal resulting in error.

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