These are usually panel mounted, which allows direct viewing of the front panel and provides a means of manual intervention by the operator, either to manually control the process or to change control setpoints. Originally, these would be pneumatic controllers, a few of which are still in use, but nearly all are now electronic.

Quite complex systems can be created with networks of these controllers communicating using industry-standard protocols. Networking allows the use of local or remote SCADA operator interfaces and enables the cascading