Software is essential in today's machine controllers for reuse, reconfiguration and cost-reduction. Formal models and specifications for such a system are critical to guarantee correctness. In this paper, we present an approach to model manufacturing control systems based on a framework for subsystem composition and a nested finite state machine model for the system behavior. The paper presents the application of model-based methodology for real control systems. Some preliminary results obtained show that development costs were reduced significantly due to the domain-specific knowledge captured in such a model.