Digital inputs are commonly configured as what?

• A. Dry contacts
• B. Voltage-sensing switches
• C. Optical sensors
• D. Relay coils

Answer: (A)

Digital inputs are commonly configured as dry contacts because they provide a simple, reliable on/off signal without adding active electronics at the sensing point. A dry contact is just a switch—two conductors that close or open to complete a circuit. The controller supplies the sensing condition (often a small voltage or pull-up) and detects the change when the contact closes. This setup offers isolation between field devices and the controller, is inexpensive, robust, and compatible with many field devices like door switches, limit switches, and pushbuttons.

Other options involve sensors that generate their own voltage or current, or devices that emit a signal (optical sensors, voltage-sensing switches), or loads driven by outputs (relay coils). These don’t provide the straightforward contact-closure interface that digital inputs expect, so they aren’t the typical configuration for digital inputs.