What are examples of passive input devices?

What are examples of passive input devices?

• A. Active sensors with integrated power supply.
• B. Devices that require external sensing fields.
• C. Digital controllers that source current into the loop.
• D. PLC input cards, remote I/O modules, and some analyzers.

Answer: (D)

In a 4-20 mA loop, passive input devices do not supply power to the loop; the loop itself powers them, and they simply sense the current or convert it to a signal for the controller. PLC input cards and remote I/O modules fit this role because they sit in the loop path to read the current and present a signal to the control system, typically by developing a voltage across a sense element that the PLC reads. Some analyzers that only monitor the loop current without providing power also count as passive inputs. The other options describe devices that provide power to the loop or require external power or fields to operate, which means they are not passive input devices.

In a 4-20 mA loop, passive input devices do not supply power to the loop; the loop itself powers them, and they simply sense the current or convert it to a signal for the controller. PLC input cards and remote I/O modules fit this role because they sit in the loop path to read the current and present a signal to the control system, typically by developing a voltage across a sense element that the PLC reads. Some analyzers that only monitor the loop current without providing power also count as passive inputs. The other options describe devices that provide power to the loop or require external power or fields to operate, which means they are not passive input devices.