SA (software architecture) technology is an important means to control the complexity of software, and to improve software quality and to support software development and reuse. It is merely adopted in the development of PLC software however. It is not more optimistic as for O-O (object-oriented) technology. The reason for this is the control software is so different form the general information system, and so many aspects must be taken into account. Concept of abstract object, which was not straightforward, is explained. It is advised that the controller and controlled object should be treated and designed as two objects. The limitation of the O-O technology in PLC software is discussed. A systematic PLC program development methodology is proposed, which adopts SA in system level, and O-O in unit machines level, and task-oriented technology in level of inside unit machines. The methodology is demonstrated by the design of control software for a material flow system. The PLC software employed pipe and filter model as its architecture, which was adapted at its sublayer. The controller objects are activated dynamically by a manager object. And message passing is employed for inter-objects communication.
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