LCladder

LCladder is an open-source software framework for ladder logic design, simulation, and verification. It provides a lightweight runtime for evaluating ladder diagrams and a graphical editor that lets users construct rung-based programs without hardware. The project is designed to aid learning, prototyping, and validation of control logic by offering deterministic execution, step-by-step debugging, and transparent state tracing.

Its architecture comprises a core simulation engine, a library of standard ladder blocks (contacts, coils, timers, counters, and function blocks), a signal model for inputs and outputs, and a plugin-based user interface. The runtime evaluates rungs in scan order, updates outputs based on block results, and records a trace of state changes for debugging. It supports exporting to a structured text representation and JSON descriptions, as well as interoperability with real or simulated I/O through simple adapters.

LCladder originated from a community-driven project in the mid-2010s, with its first public release in 2018. It is released under the MIT License and maintained by a collaborative network of developers and educational partners. Over time, it has added IEC 61131-3-compatible blocks, unit testing facilities, and integrations with Python and JavaScript scripting for custom blocks.

Typical use cases include classroom demonstrations of ladder logic, pre-deployment verification of control programs, and rapid prototyping of automation sequences. While not a substitute for hardware PLCs, LCladder provides a safe, observable environment for exploring ladder logic concepts and sharing control strategies.