Contacts & Coils Part 1 – Normally Open

With just the Normally Open Contact and Normally Open Coil – a surprising array of basic logical conditions can be represented.

Normally Open ContactNormally Open Contact. This can be used to represent any input to the control logic – a switch or sensor, a contact from an output, or an internal output.

When “solved” the referenced input is examined for an ON (logical 1) condition. If it is ON, the contact will close and allow power (logic) to flow from left to right. If the status is OFF (logical 0), the contact is Open, power (logic) will NOT flow from left to right.

Ladder Logic Normally Open Coil Normally Open Coil. This can be used to represent any discrete output from the control logic.

When “solved” if the logic to the left of the coil is TRUE, the refrenced output is ON (logical 1).

Solving a Single Rung

Rung of Ladder Logic

Suppose a switch is wired to Input1, and a light bulb is wired through Output1 in such a way that the light is OFF when Output1 is OFF, and ON when Output1 is ON.

When Input1 is OFF (logical 0) the contact remains open and power cannot flow from left to right. Therefore, Output1 remains OFF (logical 0).

When Input1 is ON (logical 1) then the contact closes, power flows from left to right, and Output1 becomes ON (the light turns ON).