A ladder diagram is a type of schematic diagram used to represent electrical control logic. It falls into the broader category of graphical programming languages and is one of two standardized graphical formats (alongside Function Block Diagrams) recognized under the international standard IEC 61131-3 for programming industrial controllers. Unlike wiring diagrams that show physical connections, a ladder diagram is purely logical, showing how a control circuit functions rather than how components are physically arranged.
How Ladder Diagrams Differ From Wiring Diagrams
The distinction matters because people often confuse the two. A wiring diagram (also called a connection diagram) shows where components physically sit and how conductors run between them. Two devices might appear right next to each other on a ladder diagram but sit 50 meters apart in the real installation. Wiring diagrams are what an electrician follows to route cables and terminate connections.
A ladder diagram strips all of that away. It shows how a circuit works logically and electrically, with no attempt to represent physical layout. The emphasis is entirely on making the control logic clear and readable. This is why ladder diagrams are classified as schematic diagrams rather than construction or installation drawings.
Why It Looks Like a Ladder
The name comes directly from the diagram’s appearance. Two vertical lines called “rails” represent the opposite poles of a power supply, typically 120 volts AC. Horizontal lines called “rungs” stretch between the rails, each one representing a unique parallel circuit branch. The result looks like a ladder, with power flowing from left to right across each rung.
On each rung, you’ll find input symbols on the left side and output symbols on the right. The inputs are drawn as “contacts” and the outputs as “coils,” borrowing directly from the language of physical relay circuits. A normally open contact appears as two parallel vertical lines (like an open switch) and allows current to pass when its condition is true. A normally closed contact has a diagonal slash through it and works in reverse, passing current when its condition is false. Coils sit at the far right of each rung and activate a device or output when the logic conditions to their left are satisfied.
Origins in Relay Logic
Ladder diagrams were originally a documentation method for relay racks in manufacturing and process control. Each physical relay, switch, and connection in a control panel was represented by a symbol on the diagram. When programmable logic controllers (PLCs) arrived, engineers adapted the same visual format as a programming language. The motivation was practical: factory engineers and technicians could write and maintain control programs without learning a general-purpose computer language like FORTRAN. The diagram on screen looked just like the relay hardware they already understood.
This graphical approach eventually became so widespread that ladder diagrams were once the only way to record PLC programs. Today they remain the most familiar method, though other programming formats exist alongside them.
How a PLC Reads the Diagram
When a ladder diagram runs on a PLC, the controller follows a continuous loop called a scan cycle. First, it reads all physical inputs (sensors, switches, buttons) and stores their current states. Then it solves every rung of the ladder logic in order from top to bottom, using those stored input values. Once all rungs have been evaluated, the controller updates its physical outputs (motors, valves, indicator lights) based on the results. This entire cycle repeats continuously, typically many times per second.
Each rung is essentially a logical equation. If the combination of contacts on a rung evaluates as true, the coil at the end energizes. If not, the coil stays off. Series contacts act like AND logic (all must be true), while parallel contacts act like OR logic (any one can be true). This makes ladder diagrams a way to represent logical equations through a visual format that mimics electrical circuits.
Where Ladder Diagrams Are Used
Ladder logic dominates industrial automation. Conveyor systems, packaging machines, and assembly lines all commonly rely on ladder-programmed PLCs to sequence their operations. Process industries use them to control temperature, pressure, and flow. Building automation systems for HVAC, lighting, and power distribution frequently use ladder logic as well. Motor control is another core application, where the start/stop/interlock sequences that protect expensive equipment translate naturally into ladder rungs.
The format’s staying power comes from its readability. A maintenance technician troubleshooting a machine at 2 a.m. can follow the logic rung by rung, checking whether each contact condition is true or false, and quickly isolate where the control sequence is breaking down. That direct, visual traceability is something more abstract programming languages don’t offer as easily, which is why ladder diagrams remain the default choice for straightforward control tasks in factories and plants worldwide.