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To reduce the difficulties in developing PLC based control software for mechanical system, an approach named task-oriented analysis and design (TOAD) is proposed. TOAD emphasizes task centric and model centric developing process. By their property, control tasks are classified as private-sequential task, global-real-time task and asynchronous-concurrent task. The classification prompts developers to find tasks and to analyze a system. Hierarchical modeling method is presented. Intertask communication and multitask scheduling method is given. The method is demonstrated using a case study of elevator control software. By encapsulating the tasks into subroutines, and by means of setting or resetting the enable input bit of the subroutines to enable or disable the subroutines to be executed or not to dispatch the tasks, a structured and time-efficient program is developed, which can be reused as a framework of the PLC software for mechanical system control.