Ladder logic

Ladder logic was originally a written method to document the design and construction of relay racks as used in manufacturing and process control. Each device in the relay rack would be represented by a symbol on the ladder diagram with connections between those devices shown. In addition, other items external to the relay rack such as pumps, heaters, and so forth would also be shown on the ladder diagram.

Ladder logic has evolved into a programming language that represents a program by a graphical diagram based on the circuit diagrams of relay logic hardware. Ladder logic is used to develop software for programmable logic controllers (PLCs) used in industrial control applications. The name is based on the observation that programs in this language resemble ladders, with two vertical rails and a series of horizontal rungs between them. Ladder diagrams were once the only way to record programmable controller programs, but today, other forms are standardized in IEC 61131-3. For example, instead of the graphical ladder logic form, there is a language called Structured text, which is similar to C, within the IEC 61131-3 standard.

Ladder logic is widely used to program PLCs, where sequential control of a process or manufacturing operation is required. Ladder logic is useful for simple but critical control systems or for reworking old hardwired relay circuits. As programmable logic controllers became more sophisticated it has also been used in very complex automation systems. Often the ladder logic program is used in conjunction with a human–machine interface (HMI) program operating on a computer workstation.

The motivation for representing sequential control logic in a ladder diagram was to allow factory engineers and technicians to develop software without additional training to learn a language such as FORTRAN or other general-purpose computer language. Development and maintenance were simplified because of the resemblance to familiar relay hardware systems. Implementations of ladder logic may have characteristics, such as sequential execution and support for control flow features, that make the analogy to hardware somewhat inaccurate.

Ladder logic can be thought of as a rule-based language rather than a procedural language. A "rung" in the ladder represents a rule. When implemented with relays and other electro-mechanical devices, the various rules execute simultaneously and immediately. When implemented in a programmable logic controller, the rules are typically executed sequentially by software in a continuous loop, or "scan". By executing the loop fast enough, typically many times per second, the effect of simultaneous and immediate execution is achieved. Proper use of programmable controllers requires an understanding of the limitations of the execution order of rungs.

The language itself can be seen as a set of connections between logical checkers (contacts) and actuators (coils). When a path exists from the left side of the rung to the output through asserted (true or "closed") contacts, the rung is considered true, and the output coil storage bit is set to 1 or (true). If no such path exists, the output is false (0), and the "coil" by analogy to electromechanical relays is considered "de-energized". This analogy between logical propositions and relay contact status was established by Claude Shannon.

Ladder logic has contacts that make or break circuits to control coils. Each coil or contact corresponds to the status of a single bit in the programmable controller's memory. Unlike electromechanical relays, a ladder program can refer any number of times to the status of a single bit, equivalent to a relay with an indefinitely large number of contacts.

So-called "contacts" may refer to physical ("hard") inputs to the programmable controller from physical devices such as pushbuttons and limit switches via an integrated or external input module, or may represent the status of internal storage bits which may be generated elsewhere in the program.