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3.3 a simple hardware system - traffic lights

The model below provides a simple hardware example, a traffic lights system:

traffic light example

Notation: The model comprises six diagrams. Each diagram represents one system as a composition of component systems. Component systems are represented in diagrams by named lines or named rectangles. There is no notational distinction between lines and rectangles. The touching of two shapes (rectangles or lines) represents an interaction between two systems so that the two systems affect each others behaviour. A set of similar component systems is represented by a shaded rectangle or shaded line. Members of such a set are distinguished with suffixes.

The system consists of a set of traffic junctions (cross roads), controlled by a traffic control mechanism, monitored by a traffic management centre. Each junction has two traffic light pairs (the two lights in each traffic light pair always display the same colour light). The traffic control mechanism includes a central control (IT) system that controls all traffic lights.

The use of the lines in the above model does have advantages. They probably make the model easier to read. The viewer of the a diagram in the model can think of the systems represented by lines as existing to provide the interactions between the systems represented by rectangles. The viewer can also think of them as less complex systems.

However this is not necessarily helpful. The system represented by the rectangle, central control system, must provide an interaction between the systems represented by the line, communications link and control panel, else the system represented by the rectangle, traffic control mechanism, will not be able to fulfill its role. In addition the system represented by the line, communications link, could be much more complex than that represented by the rectangle, central control system.

Hence it might be better to dispense with named lines and use the model:

traffic light example without lines

last revision: 21st June 1997