Which of the following is an example of a passive input device?

• A. Active transmitter
• B. PLC input cards
• C. 4-20 mA input module
• D. Open-collector input

Answer: (B)

A passive input device is one that does not supply power to the measurement loop; it only senses the signal that is already present. In a 4-20 mA control loop, the transmitter in the field powers the loop and drives the current to reflect the measured value. The PLC input cards sit at the controller end and simply detect that current or its corresponding voltage, converting it into a digital or logical signal for the PLC. They don’t generate or source the loop current themselves, which makes them passive with respect to the process signal. Open-collector inputs also don’t source power, but they’re a specific interface type used for digital signaling. The PLC input card is the clear example of a passive input device in this context because it represents the typical sensing interface that reads the externally provided signal without generating it.