Ladder Logic overview

Ladder Logic, also known as Ladder Diagram (LD), is one of five IEC standard 61131-3 languages, which include the following:

1. LD
2. Functional Block Diagram (FBD)
3. Instruction List (IL)
4. Sequential Functional Charts (SFCs)
5. Structured Text (ST)

Ladder Logic was originally a written method for capturing the wiring of relay circuits, also known as relay logic. The name Ladder Logic can be attributed to the diagrams it uses that resemble a ladder. Two vertical lines (often referred to as L1 and L2) represent the voltage of the circuit and the horizontal lines and symbols represent the devices (buttons, motors, and breakers) connected to the circuit.

Each horizontal line in the circuit is known as a rung. Once microprocessors enabled programmable logic in control systems, Ladder Logic evolved into a programming language, rather than an engineering diagram.

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