A proximity sensor connected to a PLC input is designed to detect metallic objects. The sensor has a maximum sensing distance of 20 mm. During operation, a metallic target passes within 15 mm of the sensor, but the PLC input does not register a state change. The sensor's supply voltage is confirmed to be correct. What is a possible cause?

Question: A proximity sensor connected to a PLC input is designed to detect metallic objects. The sensor has a maximum sensing distance of 20 mm. During operation, a metallic target passes within 15 mm of the sensor, but the PLC input does not register a state change. The sensor's supply voltage is confirmed to be correct. What is a possible cause?

Answer options:

• The target is moving too slowly for the sensor to detect. ✅ The sensor's output type is incompatible with the PLC input module.
• The sensor is configured for non-metallic object detection.
• The PLC ladder logic incorrectly reads the input status.

Correct answer: The sensor's output type is incompatible with the PLC input module.

Explanation: Even if the target is within the sensing range, an incompatible output type (e.g., NPN sensor to a PNP input, or current sink to current source) would prevent the PLC input from registering a valid signal. Target speed is usually a factor for very high-speed applications, and a proximity sensor is specific to metallic or non-metallic objects, not switchable in this context. Incorrect PLC logic would be the next step if hardware compatibility is ruled out.