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SpinDistribute.com Article Directory

The following article was published in our article directory on October 1, 2011.
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Ladder Logic

Article Category: Computers and Technology

Author Name: Michael Garza

Ladder logic is a programming language utilized in programming PLCs or programmable logic controllers. It is represented by graphical diagrams based on circuit diagrams using relay logic. It is called this because programs in the language resemble that of ladders. Early adopters of the programming language believed that ladder logic was a language that any engineer could easily understand and that additional training wouldn't be necessary to make use of it, since the language resembled familiar concepts with most hardware systems used by engineers and technicians. However, as more modern programming languages developed, and as ladder logic programmers realized that practical implementations of the language have characteristics that make the analogy with hardware somewhat inaccurate, that argument soon began to lose steam.

However, ladder logic continued to be used in programmable logic controllers, not only because of the ease by which they can be implemented within PLCs, but because of the easy way by which they can be modified to suit a PLC's requirement. One drawback of the language however is the inability of different model PLCs to be able to interchange programs between models. This is because each model may have a different set of ladder notations preventing the program from seamlessly being interchanged with one another.

Uses and limitations

Ladder logic is best suited when there are only two variables to deal with in processing control. Usually these variables only require a true or false answer, and as such the ladders would flow in two different routes, taper off, or lead to a null design. Ladder logic has contacts that make or break circuits, hence the necessity to compute between true and null variables.

In cases where the ladder language comes up lacking, modern technology has come up with better devices to cover up for its deficiencies. In general, microprocessors have made ladder logic a bit redundant. Other notational languages such as function blocks and sequential function charts can work with ladder logic in limited applications.

Dissecting the rungs of the ladder

The ladder has four basic variables, which are called registers. They are inputs, outputs, data, and internal relay. Inputs are marked by x's, outputs by y's, data by integers, hex, and real numbers, and internal relays by r's. What is important to remember is that a rung will have input instructions on the left side of the rung and output instructions on the right side of the rung. Relay conditionals and output coils form the basic structure of your IF THEN statement. Each input and output will also need to be addressed before they can begin to function correctly. Logic functions in the ladder make use of AND and OR functions, and perform the necessary calculations for a true and false statement.

The need to study and understand the language

In general, only systems engineers, automation scientists, and hobbyists need to study this language in detail. Most programming language can substitute most of the functions ladder logic performs. However, where most PLCs are concerned, ladder logic is still required, both to program the PLCs and to make use of it.

Keywords: automation controls, factory floor automation, plant automation, industrial automation, control systems integration, control systems design, distributed controls systems, SCADA, supervisory control and data acquisition, PLC programming, programmable logic controller, Allen Bradley, A-B, Siemens, S7, S5, WinCC, Wonderware, iFix, Factory Talk, Rockwell Automation, RS View, RS Logix, ControlLogix, Compact Logix, Kinetics, Ladder Logic, Factory Talk Transaction Manager, Human Machine Interface, Panel Views, vision systems, motion control and design

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