A limit switch is a valve actuator accessory which monitors valve position and send a discrete on/off signal to the DCS or PLC at the ends of travel of the valve. These signals are used for status, troubleshooting or for interlocking purposes.

There are 2 types of switches: mechanical and proximity. In a mechanical switch, something physically toggles the switch whereas in a proximity switch, a magnetic or metallic target causes the closure of a hermetically sealed switch.

In the most modern plants, limit switches are hooked up using multi-drop wiring and communicate over a network style protocol such as AS-i or Foundation Fieldbus.