Function blocks are equivalent to integrated circuits and represent a specialized control function. User-defined functions are allowed under the standard. IEC has defined standard functions like ADD(ition), ABS (absolute) and SQRT. Programs, functions and function blocks are called Program Organization Units. A program is configured for a specific control system, and is considered to be a network of functions and function blocks. Variables are only assigned to explicit hardware addresses in configurations, resources, or programs to achieve hardware independence from the program. Defining data types prevents such errors as dividing a Date by an Integer.

The common elements section defines data types, variables, configuration, resources, tasks, and program organization units. IEC 61131-3 has two parts common elements and programming languages. International standard, IEC 61131, was adopted to bring order from the chaos. Different conventions evolved for handling special functions leading to confusion when changing from one manufacturer to another. Editors for the traditional PLC languages have been developed specifically for particular hardware platforms. There are also specialized languages for motion control, and C is often used as a control language. RLL is the most-used programming language for machine control, but four other languages used today include function block diagram (FBD), structured text, instruction list, and flowcharts. RLL program editors began incorporating a library of special function blocks for timers, counters, math, messaging, and increasingly complex functions like PID control. This is fine for some machine functions, but what happens when the programmer needs special functions, wants to do math, has analog inputs and outputs, and wants to communicate? Function blocks solved this problem. These diagrams looked like relay logic diagrams familiar to electricians, while representing digital logic. Relay ladder logic (RLL), also called ladder diagrams (LD), were found to be ideal representations of Boolean logic (see diagram). A statement would be something like вЂIf (limit switch contact) A closes OR (motor starter contact)B closes AND (limit switch contact) C is closed THEN energize coil D.вЂ™ There were, and still are, text-based languages to accomplish this programming. PLC programming was originally strictly Boolean.

Faster, more powerful controller microprocessors and MicrosoftвЂ™s Windows-based graphic environment have combined to allow programmers to accomplish complex tasks. Programming editors and languages are increasingly loaded with powerful functions.

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