Glossary · PLC and control programming
Ladder diagram (LD)
Also known as: LD, Ladder logic diagram
German: Kontaktplan (KOP)
In IEC 61131-3, the ladder diagram (LD) is a graphical programming language that represents Boolean logic as contacts and coils placed on rungs between a left and a right power rail, derived from relay circuit diagrams.
- PLC programming
- Standards
In one sentence
Ladder diagram (LD) is a graphical IEC 61131-3 language that draws logic as contacts and coils on rungs between power rails.
Example
A rung with a normally open contact Start in parallel with the holding contact Motor, in series with a normally closed contact Stop, drives the coil Motor.
How it applies
- Engineering: LD is well suited to discrete logic, interlocks and start/stop circuits. It can also call functions and function blocks as boxes on a rung. For calculations and data handling, text languages are usually clearer.
- Maintenance: LD is especially common in North America and among electricians, because it resembles the relay schematics they know. Online status shows power flow along the rung, which makes faults easy to trace.
- Safety: A software interlock in LD is not a Hardwired interlock. Whether a function implemented in LD can take on a safety role depends on the controller and on the evidence required by the applicable functional safety standard.
- Documentation: Give rungs meaningful comments and use symbolic names that match the electrical drawings, so that technicians can move between schematic and program without translation tables.
Ladder diagram vs. ladder logic
Ladder diagram (LD) is the language defined in IEC 61131-3. Ladder logic is the broader, older practice of expressing control logic in relay-ladder form, which predates the standard and appears in many vendor-specific dialects.