ISPSoft User Manual

Chapter 7 Function Block
In view of this, the concept related to function block instances is introduced into IEC 61131-3.
After users add a POU of the function block type, declare local symbols in the POU, and write a
program in the POU, they will get a function block definition. The function block definition is like a
document. It does not participate in any operation, and does not appropriate any resources that the
PLC uses.
If a POU of the program type wants to call this function block, the users must declare a symbol
whose data type is a function block in advance. In other words, the users have to produce an object
which takes part in an operation according to the function block definition. The object produced is a
function block instance.
When the program is compiled, the system assigns a substantial memory block to the function block
instance and the local symbols in the function block instance according to the definition.
If the same operation is required, and a different memory block is also required, users can make use
of the characteristic of a function block instance. After a different symbol is declared for a function
block which requires a different memory block, the system will regard the different symbol as a
different function block instance. When the program is compiled, a different memory block is
assigned to the different function block instance.
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