Which modules in a control system are designed to have opto-isolation between field power and controller power?

• A. Input modules
• B. Output modules
• C. Both input and output modules
• D. Power supplies

Answer: (C)

Opto-isolation is used to separate the controller’s electronics from the field wiring to prevent noise, spikes, and differences in ground potential from affecting the controller. In a control system, both input and output modules are designed with this isolation so signals can be transferred without a direct electrical path between field power and controller power.

For input modules, the field signal from sensors is fed through an opto-isolator, so the controller sees a safe, low-voltage representation while the field side and controller side stay galvanically isolated. This protects the controller from transients or faults on the sensor side.

For output modules, the controller’s low-voltage signal drives the LED side of an opto-isolator, and the photodetector on the field side switches the higher-power load. The isolation keeps the controller safe from faults or noise on the actuator side and prevents ground differences from propagating into the controller.

Power supplies aren’t the primary place where this isolation is implemented between field and controller power in typical modular I/O configurations, making input and output modules the correct answer.