Real-time systems are those that must produce correct responses within a definite time limit. If computer responses exceed these time bounds, then performance degradation and/or malfunction results. Examples of real-time systems are the cruise-control system in an automobile, a security system, and missile-control systems. The Ward and Mellor data flow diagram creates the finer distinctions needed to show the flow of control as well as the flow of data on the diagram.
The methodology for analyzing and designing real-time systems is an extension of process modeling, the familiar DFD graphic language used by Yourdon (see Yourdon/DeMarco data flow diagrams), DeMarco, or Gane and Sarson (see Gane & Sarson data flow diagrams). The data flow diagram is used, including the symbol shapes for processes, data stores, and data flow arrows.
In general, most process models of systems start with a context diagram, on which a process symbol, some external entities, one or more data stores, and some data flows are pictured. In a Ward and Mellor context diagram, the symbol traditionally labeled “The System” could be a data transform or a control transform. A cruise-control system, for example, would more typically be involved with transforming control rather than data.
That symbol expands to a “Child” diagram which can have 5 to 9 data and control transform symbols, each of which can expand to a child diagram, and so on, in an analysis process known as creating a set of leveled diagrams: see Creating a set of leveled diagrams. In the real-time methodology, the child diagram of a control transform is likely to be a Ward and Mellor state transition diagram: see Ward and Mellor state transition diagrams.
