Ladder Logic, also known as Ladder Diagram (LD), is a graphical programming language widely used for Programmable Logic Controllers (PLCs). Standardized under IEC 61131-3, it originates from the relay logic diagrams used in manufacturing control systems. Because the visual layout resembles electrical schematics, it allows electricians and plant technicians to program and troubleshoot controllers without requiring extensive knowledge of text-based coding languages.
The program structure mimics a ladder with two vertical rails representing power lines and horizontal "rungs" containing the logic circuits. The logic executes sequentially from left to right and top to bottom. Common symbols include inputs represented as contacts (-| |- or -|/|-) and outputs represented as coils (-( )-). When the input conditions on a rung are met, the output is energized, simulating the current flow in a physical relay circuit.
While originally designed for simple Boolean logic, modern Ladder Logic includes advanced functions such as timers, counters, mathematical operations, and data manipulation. It remains a standard method for integrating industrial equipment, including DC power supplies and high-voltage systems, into automated production lines.