Which statement about field devices in relation to PLC hardware is true?

• A. They are not part of the PLC hardware.
• B. They are part of the PLC hardware.
• C. They are software components.
• D. They are used only in fixed PLCs.

Answer: (A)

Field devices are the sensors and actuators located in the process environment, while the PLC hardware is the controller itself plus its I/O modules and power components in the control cabinet. The field devices provide the real-world signals the PLC reads (like a temperature sensor or pressure switch) and the signals it sends to actuators (like a valve or motor starter). They connect to the PLC through input/output modules or fieldbus networks, but they do not reside inside the PLC hardware. This separation is what lets the PLC control a system while the field devices handle the physical measurements and actions in the plant. The relationship holds for both fixed and modular PLCs—field devices remain external to the PLC hardware and interface with it rather than being part of it.