Master the Magic of PLCs in 2026 – Conquer the PMMI Programmable Logic Controllers Challenge!

In ladder logic, a contact is a condition that can be true or false. It acts like a switch: it passes the rung’s logic when its state is satisfied (with normally open contacts closing when the input is on, and normally closed contacts opening when the input is on). It doesn’t energize anything by itself; it simply gates the flow of current through the rung.

A coil is an output device. When the rung’s conditions are all true, the coil is energized, which activates the associated output (such as a lamp, motor, relay coil, or an internal bit). The coil represents the action taken as a result of the rung evaluating to true, not a condition itself.

So the contact is about evaluating a condition; the coil is about performing the action of energizing an output.