Ladderdiagram

Ladder diagram is a graphical programming language used to program programmable logic controllers (PLCs). It is derived from relay logic and uses a ladder-like schematic with two vertical rails and horizontal rungs. Each rung encodes a logic condition that controls an output coil.

Symbols include contacts that represent input conditions and coils that actuate outputs. Contacts can be normally open (NO) or normally closed (NC). Rungs may also include timers, counters, set/reset coils, and function blocks. In a rung, elements are arranged in series to represent AND logic or in parallel to represent OR logic.

In operation, a PLC scans the ladder diagram from top to bottom. For each rung, it evaluates the conditions from left to right; if the rung is true, the associated coil is energized, producing the corresponding output. The ladder is then refreshed in a cyclic scan.

Ladder diagrams originated from relay-based control systems in the mid-20th century and were adopted as a formal PLC programming language. They are standardized within IEC 61131-3 as Ladder Diagram (LD), alongside function block, structured text, and other graphical languages.

Ladder logic is valued for its readability and direct hardware mapping, making it well suited for simple, discrete control tasks. It can become difficult to manage for complex logic, and many programmers use other IEC 61131-3 languages for more advanced processing.