Free Ladder Logic Maker

Ladder logic is the dominant programming language for Programmable Logic Controllers (PLCs) in industrial automation. It represents control logic as a series of horizontal rungs — resembling the rungs of a ladder — where each rung is a logical condition that must be satisfied to energize an output. Originally designed to replicate the look of relay-based control panels, ladder logic is now governed by the IEC 61131-3 standard and is used to document, design, and communicate PLC programs across the manufacturing, process control, and building automation industries.

Ladder logic was developed in the 1960s and 1970s as PLCs began replacing hard-wired relay panels in manufacturing. The graphical representation deliberately mimics the appearance of a relay ladder diagram — two vertical power rails connected by horizontal rungs — so that electrical engineers and technicians already familiar with relay schematics could read and write PLC programs without learning an entirely new language. This heritage means ladder logic remains the most widely understood PLC programming language on factory floors worldwide.

The IEC 61131-3 standard, first published in 1993 and updated since, formally defines ladder logic alongside four other PLC programming languages: Function Block Diagram, Structured Text, Instruction List, and Sequential Function Chart. Ladder logic is the most common of the five for discrete manufacturing and motor control applications. Each rung in a ladder program is evaluated left to right, top to bottom, in a continuous scan cycle.

The fundamental elements of ladder logic are contacts and coils. A normally open contact (XIC — Examine If Closed) passes power when its associated bit is set to 1 (true). A normally closed contact (XIO — Examine If Open) passes power when its associated bit is 0 (false). An output coil (OTE — Output Energize) is set to 1 when power flows through the entire rung from left to right. These three elements alone can express any combinational logic, and they are extended by timer instructions (TON, TOF, RTO), counter instructions (CTU, CTD), and comparison and math instructions for more complex control requirements.

A seal-in or latching circuit is one of the most important patterns in ladder logic. When an operator presses a momentary start button, a single rung contact closes briefly. A seal-in contact — wired in parallel with the start button contact — captures that momentary pulse and holds the output energized even after the start button is released. A stop button, wired in series as a normally closed contact, breaks the rung continuity and de-energizes the output. This start/stop/seal-in pattern is the foundation of nearly every motor control circuit in industrial automation.