Input wire connector Value Wire Value Input reference 384 Part 6 Function Block Programming Part Objectives After completing this part, you will be able to: •• Describe the difference between ladder logic and function block diagram programming •• Recognize the basic elements of a function block diagram •• Write and read a function block diagram Function Block Diagram (FBD) A function block diagram (FBD) is a graphical depiction of process flow using simple and complex interconnecting blocks. It is similar to a ladder logic diagram, except that function blocks replace the interconnection of contacts and the coils. In addition, there are no power rails. A function block circuit is analogous to an electrical circuit where links and wires depict signal paths between components. The workplace is known as a sheet and consists of function blocks joined together with lines called wires. The structure of a function block program, or routine, is shown in Figure 15-95. A function block diagram consists of four basic elements: function block, references, wire connectors, and wires. Data flows on a wire from wire connectors or input references, through the function block, and then is passed on to an output reference. The line type of the link between function blocks indicates what type of data is present. A dash line indicates Function block Value Value Value Value Integer or real Boolean Output reference (0 or 1) Figure 15-95 Structure of function block or routine. a Boolean signal path (e.g., 0 or 1) and a solid line indicates an integer or real value. Function blocks are graphical representations of executable code. A function block can take one or more inputs and make decisions or calculations and then generate one or more outputs. There are many different types of function blocks included in the programming software to perform various common tasks. In addition, customized Add-On instructions can be created by the programmer for sets of commonly used logic. Once an Add-On instruction is defined in a project, it appears on the instruction toolbar and behaves like the standard instructions. Figure 15-96 shows an example of a BAND (Boolean AND) function block. The information associated with a function block can be summarized as follows: • Inputs are shown entering from the left and outputs exiting on the right. • The function block type is shown within the block. • A tag name for the block is placed above it.
Petruzella_ProgrammableLogicControllers__5e
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