Five Types of PLC Programming languages

Although it seems each model of PLC has its own idiosyncratic standard for programming, there does exist an international standard for controller programming that most PLC manufacturers at least attempt to conform to. This is the IEC 61131-3 standard, which will be the standard

One should take solace in the fact that despite differences in the details of PLC programming from one manufacturer to another and from one model to another, the basic principles are largely the same.

There exist much greater disparities between different general-purpose programming languages (e.g. C/C++, BASIC, FORTRAN, Pascal, Java, Ada, etc.) than between the programming languages supported by different PLCs, and this fact does not prevent computer programmers from being “multilingual.”

I have personally written and/or analyzed programs for over a half-dozen different manufacturers of PLCs (Allen-Bradley, Siemens, Square D, Koyo, Fanuc, Moore Products APACS and QUADLOG, and Modicon), with multiple PLC models within most of those brands, and I can tell you the differences in programming conventions are largely insignificant.

After learning how to program one model of PLC, it is quite easy to adapt to programming other makes and models of PLC.

PLC Programming languages

The IEC 61131-3 standard specifies five distinct forms of programming language for industrial controllers:

• Ladder Diagram (LD)
• Structured Text (ST)
• Instruction List (IL)
• Function Block Diagram (FBD)
• Sequential Function Chart (SFC)

Not all programmable logic controllers support all five language types, but nearly all of them support Ladder Diagram (LD), which will be the primary focus of this book.

Programming languages for many industrial devices are limited by design.

One reason for this is simplicity: any programming language simple enough in structure for someone with no formal computer programming knowledge to understand is going to be limited in its capabilities.

Another reason for programming limitations is safety: the more flexible and unbounded a programming language is, the more potential there will be to unintentionally create complicated “run-time” errors when programming.

The ISA safety standard number 84 classifies industrial programming languages as either Fixed Programming Languages (FPL), Limited Variability Languages (LVL), or Full Variability Languages (FVL).

Ladder Diagram and Function Block Diagram programming are both considered to be “limited variability” languages, whereas Instruction List (and traditional computer programming languages such as C/C++, FORTRAN, BASIC, etc.) are considered “full variability” languages with all the attendant potential for complex errors.