ISPSoft User Manual 5.1 Program Organization Units and Tasks Program Organization Units (POUs) and tasks are important programming concepts related to IEC 61131-3. They upgrade programming from traditional program writing to project management. The difference between traditional ladder diagrams and IEC 61131-3 is described below. Traditional ladder diagrams and IEC 61131-3 are represented by the following architectures. In the classic architecture, users write a source code. In the IEC 61131-3 architecture, a program is divided into several program organization units. Every program organization unit can be developed independently, and can be assigned a task.
If a control system reaches a certain scale, the internal control procedures will be quite complex, and the relation between the procedures will be closer. If the classic architecture is adopted, a lot of time and manpower must be invaded in developing and maintaining a program. However, in the IEC 61131-3 architecture, the program organization units are reusable and modular. Besides, the program organization units can be integrated easily by means of the management of the tasks. The concepts related to program organization units and tasks will be introduced in the following section. 5-2 Traditional ladder diagram IEC 61131-3 Main program 1.
A single program is divided into several independent program organization units. 2. A program organization unit which has been created is assigned to a cyclic task. General Subroutine 1. A subroutine is a function block. 2. A function block which has been created can be called by a program organization unit. Interrupt subroutine 1. Different interrupt subroutines are independent program organization units. 2. A program organization unit which has been created is assigned to an interrupt task.