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Definiing requirements for process control systems is a complex task. Specification documents for such systems typically contain a mixture of engineering drawings, control loop diagrams, and narrative text. Because this type of document lacks an overall organizational framework, checking for completeness, correctness, and consistency is extemely difficult. Structured Analysis provides a well-developed framework for describing requirments for typical `data processing' systems. Organized around a `data flow diagram', the Structured Analysis Requirements document clearly describes data interfaces, transformation of intput data into output data, and data storage requirements. Unfortunately, process control systems have complex `dynamic' requirements interrelated with data processing requirements. For example, data may be continuously available but processed only at critical times; or input data to a system may have to be `locked out' based on a complex combination of external and internal conditions. Traditional Structured Analysis cannot describe these dynamic requirements effectivly. This paper describes an extension of Structured Analysis to describe process control system requirements. The specification document is organized around two interrelated `pictures' of the system with a `state transition diagram' supplementing the data flow diagram to clarify system dynamics. The technique has been sucessfully applied to a number of systems, including some involving simultaneous control of a number of interrelated processes. The use of the technique is illustrated on a bottling system. The example draws on elements from several systems successfully modelled by these techniques.