Ladder Logic: Here are the Basics

What is Ladder Logic? A Definition

Ladder logic stems from the history of relays. At one time, relays were the primary control for most automatic systems. These electromechanical devices consisted of coils and contacts that they moved. Energized coils moved their contacts from their resting position to their active position (either closed to open or open to closed.)

Close up image of a real ladder and a light bulb.  Visual representation of the inspiration basic ladder logic programming can allow for.
Ladder logic is shaped like an actual ladder, and is read from top to bottom/left to right.

In practical application, a ladder diagram showed how to wire relays together. This diagram looked like a drawing of a standard household ladder with uprights and rungs. Modern-day ladder logic still follows these conventions. Relays did (and still do) their job well, but can be cumbersome due to the sheer size of multiple relays wired together. This is where programmable logic controllers using ladder logic can be advantageous: able to do the same kind of job but in significantly less space.

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