Which statement about 2-wire transmitters is true?

• A. They require a separate power supply for each sensor.
• B. They have no noise immunity.
• C. They are reliable for long cable runs.
• D. They output only voltage signals.

Answer: (C)

2-wire transmitters are powered from the same 4-20 mA loop they communicate over, so they don’t need a separate power supply. The actual signal is carried as current in the loop, not as a voltage on a separate wire. That current-based approach is well-suited to long distances because current is less affected by resistance and wiring losses than voltage, so you can run longer cables as long as the loop can maintain the required 4–20 mA range. This makes 2-wire devices reliable for remote sensing points and long cable runs.

The other statements don’t fit: 2-wire devices do require no separate supply (they’re loop-powered); they do have noise immunity due to the current loop’s characteristics (not “no noise immunity”); and they do not output only voltage signals (they output current).