Glossary · LD
What is Ladder logic (LD)?
A graphical PLC language drawn like relay circuits: rails, contacts and coils. The most widely used language in maintenance.
It reads left to right: contacts are the conditions and the coil at the end is the output. Maintenance teams prefer it because it looks like the wiring diagram and is easy to follow online.
Its limits show up with maths, loops and data handling, which is where Structured Text wins. Most projects use ladder for interlocks and safety, ST for maths and sequences.
Related terms
- Structured Text (ST)
- A textual programming language in IEC 61131-3 resembling Pascal, used for complex PLC logic, calculations and state machines.
- IEC 61131-3
- The international standard that unified PLC programming: five languages, the data types, the program organization units and the standard function blocks.
- Seal-in (latch)
- A pattern that keeps an output running after a momentary button press, by feeding the output back into its own start condition.