What is a 4-wire transmitter?

• A. A transmitter that uses a separate power supply for its internal electronics.
• B. A transmitter powered entirely by the loop signal.
• C. A transmitter with two outputs.
• D. A transmitter that can only output voltage.

Answer: (A)

In a 4-wire transmitter, the electronics are powered from a separate supply, independent from the signal loop. Two conductors provide power to the transmitter’s internal circuitry, while two other conductors carry the 4–20 mA output to the process. This separation supports stable operation and electrical isolation between the transmitter’s circuitry and the loop power, and it allows the transmitter to function even if the loop supply isn’t delivering power to the electronics. The loop current remains the signal path, not the power path, which is why such transmitters are described as self-powered or independently powered. The other statements don’t fit because loop-powered devices derive power from the loop, a 4-wire arrangement isn’t defined by having two outputs, and a typical transmitter in this context outputs current (4–20 mA) rather than being limited to voltage.