What is Ladder Logic Programming?

Ladder logic programming is a special programming language which represents a program by a graphical diagram. It is based on the circuit diagrams of relay-based logic hardware. The primary use of ladder logic programming is to develop software for programmable logic controllers (PLC’s) that are used in industrial control applications. The program name “ladder logic” is based on the observation that programs in this language resemble ladders, as it consists of two vertical rails and a series of horizontal rungs between them.

Where is Ladder Logic Programming applied?

Ladder logic is used widely in the programming of programmable logic controllers due to PLC’s requirement for sequential control of a manufacturing or process operation. This is useful for reworking old hardwired relay circuits, as well as simple but critical control systems. As programmable logic controllers became more sophisticated over time, ladder logic programming has also been applied in very complex automation systems.

Ladder Logic Programming varies according to Programmable Logic Controllers

The manufacturers of programmable logic controllers also provide associated ladder logic programming systems. It is interesting to note that the ladder logic languages from two different manufacturers are not completely compatible; programming is thus a set of closely related programming languages rather than one language. The same principle applies to programmable logic controllers; programmable logic controllers within the same family may have a different ladder notation to prevent programs from being interchanged seamlessly between models.

How does Ladder Logic Programming work?

Ladder logic programming is seen as a rule-based language, rather than a procedural language. This is because each “rung” in the ladder represents a rule. When the various rules are implemented with relays and other electromechanical devices, they “execute” immediately and simultaneously. When the rules are implemented in a programmable logic controller, the rules are being executed sequentially by software in a continuous loop or scan. When this loop or scan is executed fast enough (many times per second), the effect of immediate and simultaneous execution is achieved to within the tolerance of the time required to execute every rung in the loop. This is also known as the “scan time”. The working of ladder logic programming is thus similar to other rule-based languages which include SQL or spreadsheets. However, it should be remembered that the proper use of programmable logic controllers requires an understanding of the limitations of the execution order of the ladder logic rungs.