Function Block Diagram PLC programming


The IEC (International Electrotechnical Commission) identifies five standard programming languages as the most common for both discrete programmable controllers and process. They are LD (Ladder Diagram), FBD (Function Block Diagram), SFC (Sequential Function Chart), IL (Instruction List), ST (Structured Text). In this article we will describe on the Function Block Diagram (FBD) programming language.

Although Ladder Programming may be the most widespread language, a survey conducted by Control Engineering magazine several times ago highlighted growth in the use of programming languages other than Ladder. Function Block Diagram is a sample. Even though the adoption rate for this language has slowed relative recently to other languages such as Structured Text, Function Block Diagram Programming is probably the second most used language widely.

This graphical language is resembling a wiring diagram even more so than Ladder code. The blocks are wired together into a sequence that is easy to follow with Function Block Diagram. It uses the same instructions as Ladder Diagram, but visually is more understandable to a viewer who is not versed in relay logic.

The major advantage is that programs written in the Function Block Diagram tend to be easy to follow, just follow the path. This programming language is ideal for simpler programs containing of digital inputs such as sensors of photoelectric, and outputs such as manifolds of valve, and could be appropriate in any applications where Diagram Ladder is ideal.

However this programming language is not ideal for large programming using special functions and I/O. the large amount of the space screen required by this style of programming language can make quickly a program unwieldy if it reaches any substantial size. And also writing a program using Functional Block Diagram requires more preparation upfront to understand the program and how the program will flow before any code is need to write into it, since it can be more difficult to make corrections later.

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