<Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (May 2012) (Learn how and when to remove this template message) </Td> </Tr> <P> 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 . See relay logic . </P> <P> 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 .). </P> <P> 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 . </P>

What type of control system logic did plc ladder logic replace