TABLE OF CONTENTS This chapter describes Function Block Diagram, the first graphical language of the IEC languages in the set:
Structured Text, Function Block Diagram, Ladder Diagram, Instruction List and Sequential Function Chart.
Function Block Diagram or FBD can be used to express the behaviour of functions, function blocks and programs as a set of interconnected graphical blocks. It can also be used within the Sequential Function Charts to express the behaviour of steps, actions and transitions.
FBD is based on viewing a system in terms of the flow of signals between processing elements. This is very similar to signal flows depicted in electronic circuit diagrams.
In this chapter we will review:
Common graphical symbols and conventions that apply to FBD and also to the other graphical languages, i.e. Ladder Diagram and Sequential Function Chart;
How to depict both simple and complex expressions;
How to show signal flow between functions and function blocks;
How to assign values to variables;
How to handle signal feedback;
The rules for function block evaluation;
Restrictions on the portability of code between ST and FBD.
The standard defines types of lines and interconnections for all the graphical languages, i.e. for Function Block Diagram, Ladder Diagram and Sequential Function Chart. IEC 1131-3 allows normal characters such as _ and ?to be used to depict graphical objects; this is called semi-graphical representation. However, the standard also allows full graphical representation. The use of full graphics has been adopted by most of the current implementations...
