The Ladder Language, also known as Ladder Logic or Ladder Diagram (LD), is a graphical programming language that is widely used to program Programmable Logic Controllers (PLCs). Its design was based on the electrical relay logic diagrams that were used in manufacturing and process control systems before the advent of PLCs. This was a deliberate choice to make it easier for electricians and plant technicians, who were already familiar with relay schematics, to transition to programming the new electronic controllers without needing to learn complex, text-based programming languages.
A ladder logic program is represented visually as a diagram that resembles a ladder with two vertical rails and horizontal "rungs" between them. The rails represent the power supply lines. The rungs contain the logic of the program, consisting of input conditions (represented by symbols that look like electrical contacts, -| |- or -|/|-) and output actions (represented by symbols that look like relay coils, -( )-). The logic is "solved" from left to right and top to bottom. A rung's output is "energized" if the combination of its input conditions is true, mimicking the flow of current in a physical relay circuit.
While it was designed for simple boolean logic, the language has been extended to include more advanced functions like timers, counters, mathematical operations, and data manipulation, making it a powerful tool for industrial machine control.