Editing Ladder logic (section)

{{Short description|Programming language for industrial controllers}}
{{about|the programming language|the FIRST competition game|Ladder Logic}}
{{refimprove|date=May 2012}}

'''Ladder logic''' was originally a written method to document the design and construction of [[relay logic|relay racks]] as used in manufacturing and [[process control]].<ref>{{cite web |url=http://ecmweb.com/archive/basics-ladder-logic |title=The Basics of Ladder Logic |quote=Ladder logic uses switch or relay contacts to implement Boolean expressions. In years past, ladder logic was made possible with discrete relays and was sometimes termed relay logic.}}</ref> 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 diagram]]s of [[relay logic]] hardware. Ladder logic is used to develop software for [[programmable logic controller]]s (PLCs) used in industrial control applications. The name is based on the observation that programs in this language resemble [[ladder]]s, 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.