ISPSoft User Manual

ISPSoft User Manual
7.1 Knowing Function Blocks
Function blocks (abbreviated as FB) play an important role in the writing of the program in a PLC.
They are also supported by IEC 61131-3. Owing to the characteristics and advantages of function
blocks, ISPSoft provides much support and many functions for function blocks.
7.1.1 Introduction of Function Blocks
A function block is a component in a program which performs an operation. It is a type of POU, but it
can not operate by itself. After a POU of the program type calls a function block, and sends the
related parameters, the function of the function block can be executed. After the execution of the
function block is complete, the internal operation result will be sent to the device or symbol specified
by the superior POU (caller).
A function block is shown below. The appearance of a function block is similar to that of an applied
instruction in that there are input pins and output pins. The usage of a function block is also similar
to that of an applied instruction. A function block is different from an applied instruction in that users
have to declare a symbol whose data type is a function block in the superior POU (caller) which calls
the function block. Besides, a function block can call another function block.
A function block is a memory unit. Every function block is assigned a substantial memory block
where the values of the internal symbols are stored. Owing to the fact that the value of the symbol in
a function block will be retained after the function block is executed, every execution result is
affected by the last execution result. In other words, the output values may be different even if the
input values are the same.
An application example of a function block is shown in the figure below. There is a symbol whose
data type is a function block in the local symbol table in the superior POU (caller). The name in the
Type… cell is the same as the name of the function block. Please refer to the following sections for
more information about the usage of a function block.
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