In a fixed and modular PLC, which interfaces provide the link between the PLC and external devices such as switches and sensors?

• A. Input
• B. Output
• C. Communication
• D. Power

Answer: (A)

The input interface is the path that connects the PLC to field devices like switches and sensors. It provides the electrical connection and signal conditioning needed for the PLC to read the state or value coming from those devices. In fixed PLCs, that path is built into the unit; in modular PLCs, you add input modules on the input side. These modules handle digital inputs (on/off signals from switches or pushbuttons) and analog inputs (from sensors giving variable values) and convert them into data that the PLC's processor can use. The other interfaces serve different roles—output interfaces drive actuators, communication interfaces link the PLC to networks or other controllers, and power interfaces supply the necessary electrical power.