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.The above realizes the function: Run = (NOT Emergency Stop) AND (NOT Stop) AND (Start OR Run) 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. While ladder diagrams were once the only available notation for recording programmable controller programs, today other forms are standardized in IEC 61131-3 (For example, as an alternative to the graphical ladder logic form, there is also a more assembly language like format called Instruction list 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 an 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 electromechanical 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). If a path can be traced between the left side of the rung and the output, through asserted (true or 'closed') contacts, the rung is true and the output coil storage bit is asserted (1) or true. If no path can be traced, then the output is false (0) and the 'coil' by analogy to electromechanical relays is considered 'de-energized'. The analogy between logical propositions and relay contact status is due to Claude Shannon.