Limitations and Successor Languages
Ladder notation is best suited to control problems where only binary variables are required and where interlocking and sequencing of binary is the primary control problem. Since execution of rungs is sequential within a program and may be undefined or obscure within a rung, some logic race conditions are possible which may produce unexpected results; complex rungs are best broken into several simpler steps to avoid this problem. Some manufacturers avoid this problem by explicitly and completely defining the execution order of a rung, however programmers may still have problems fully grasping the resulting complex semantics.
Analog quantities and arithmetical operations are clumsy to express in ladder logic and each manufacturer has different ways of extending the notation for these problems. There is usually limited support for arrays and loops, often resulting in duplication of code to express cases which in other languages would call for use of indexed variables.
As microprocessors have become more powerful, notations such as sequential function charts and function block diagrams can replace ladder logic for some limited applications. Very large programmable controllers may have all or part of the programming carried out in a dialect that resembles BASIC or C or other programming language with bindings appropriate for a real-time application environment.
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