Editing Ladder logic (section)

==Overview==
[[Image:Ladder diagram.png|thumb|right|250px|Part of a ladder diagram, including contacts and coils, compares, [[timers]] and [[monostable multivibrator]]s]]
Ladder logic is widely used to program [[programmable logic controller|PLC]]s, 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 [[Electrical wiring|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 logic|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.<ref name="Kamen99">Edward W. Kamen ''Industrial Controls and Manufacturing'', (Academic Press, 1999) {{ISBN|0123948509}}, Chapter 8 ''Ladder Logic Diagrams and PLC Implementations''</ref>  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.