Chapter 5 POU and Task 5.2 Program Organization Units 5.2.1 Program Architecture and Types Program organization units are basic elements of a program in a PLC. They are different from a traditional program in a PLC. The characteristic of the program architecture introduced by IEC 61131-3 is that a program is divided into several units. These units are called program organization units. size of a program becomes larger, the maintenance of the program and the debugging of the program will be a In the classic architecture, a source code for a PLC is composed of all procedures, including subroutines. If the burden.
In the IEC 61131-3 architecture, a program is divided into several units according to the functions or characteristics. It is convenient to develop and maintain a program. Besides, owing to the fact that program organization units are modular, different program organization units can be developed by different designers. It benefits the distribution of manpower and the execution of the project. There are two types of POUs in ISPSoft. They are programs (PROGs), function blocks (FBs) and Function (FC). Program (PROG) A POU of the program type plays a role according to the task to which it is assigned.
If a program POU is assigned to a cyclic task, the POU of the program type acts as a main program. If the POU of the program type is assigned to an interrupt task, the POU of the program type acts as a subroutine. Besides, a POU of the program type can call a function block (FB). Function block (FB) A static symbol can be declared in a function block (FB). As a result, the value of the symbol after an operation can be retained. Owing to the fact that the operation is performed on the value memorized in the function block and an input value, the output values may be different even if the input values are the same. Besides, a function block can call another function block.
Function (FC) Each function is an individual program unit assigned with actual parameters (numerical values) based on its definition. Contrary to Programs and Function Blocks, Functions (FC) have no memory and the previous function result cannot be stored, resulting in a same outcome from a function with same input values, such as performing arithmetic operation. This feature currently only supports AS5xx and DVPxxMC models. The relation among POUs is shown below. 5-3