Chapter 7 Function Block and Library If the same POU of the function block type is created a gain, the execution result is correct although different memory blocks are assigned to the local symbols in the two POUs of the function block type when the program is compiled. However, the same work is repeated when users write the program. 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. 7-19