Ladder Logic.

Ladder logic is the oldest of the IEC 61131-3 controller languages and still the most widely used, because it was designed in the 1970s to look exactly like the relay schematics the electricians of the day already read. Input contacts down the left, output coils on the right, current flowing across each rung. That heritage is the whole explanation for its staying power. Ladder is poor at arithmetic, awkward for long sequences, and clumsy for reusable code, but it is unmatched at letting a maintenance technician stand in front of a running machine in the middle of the night and trace why an output is not energizing, contact by contact, the same way they would trace a relay panel. Decades on, the workforce that maintains and modifies plant logic is still trained ladder-first, and that troubleshooting accessibility is a real operational asset rather than nostalgia. Modern practice does not write ladder-only. It writes ladder-mostly, mixing structured text for the math, function block for control loops, and sequential function chart for state machines, while keeping the I/O handling and the simple interlocks in ladder where the maintainer can read them. Safety-rated variants exist on most platforms, restricted to a certified subset of instructions. The connection to the controls dataset is direct on import. A clean I/O list with correct signal classes is what populates the controller tag database that the ladder program addresses, and a tag imported as the wrong class is a rung that will never behave as drawn.