Ladder LOGIC

Ladder Logic is an instruction set to provide services of real time, I/O, user interface, and similar services. These services are associated with the special requirements of the PLC applications domain. Because Ladder Logic is targeted toward special applications, it provides features that are compatible with real-time control application requirements. These features, when used correctly and appropriately can contribute to the safe operation of the program.

The origin of Ladder Logic is the Relay Ladder Logic notation which was first introduced to represent combinations of contacts and coils of relays using specific notation. These combinations implemented logical functions (e.g., AND or OR). The introduction of PLCs transformed Ladder Logic from a hardware design notation to a high level language, specialized for process and logic control. The Ladder Logic language, in the case of the PLC, is not the traditional limited Ladder Logic implemented with relays, but an advanced language supported by the numerical capabilities of the processor, while the Ladder Logic notation serves only a graphical user interface. Ladder Logic supports all types of programming structures from advanced subroutines, parameter passing, loops, mathematical functions, proportional plus integral plus derivative (PID) controllers, I/O calls, timers, and any other features of a high-level language. Although much changed from their original purpose and implementation, current forms of Ladder Logic are still similar to relay logic, allowing electrical engineering personnel who have traditionally have been in charge of factory automation to review and understand the code. This is an important advantage throughout the development process.

Ladder Logic is not a formally defined programming language. Each manufacturer has its own variation of Ladder Logic. In addition, many of the features associated with programming the PLC are not features of Ladder Logic itself, but the programming environment, the "shell," and the firmware mentioned above. The variety of ladder logic implementations is due to the strong coupling between software and hardware dictated by the requirements of the industrial control applications domain.