Specification of Reactive Hardware/Software Systems - Electronic ...

42 Concepts for Analysis, Specification and Design
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Transport of personalised product information objects
Feeder Feeder Feeder Wrapper Separator Stacker
Feeder
Controller
Figure 3.5: Mailing Machine with Product Information
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Feeder
Controller
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Feeder
Controller
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Wrapper
Controller
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Separator
Controller
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Stacker
Controller
Figure 3.6: Plug-and-play Structure of Controllers of Mailing Machine
3.6.4 Graphical Modelling
The essence of SHE is that complexity is handled by modelling a system as a collection
of collaborating process objects. Process objects can perform their behaviour concurrently.
They communicate solely by exchanging messages (via channels). Coherent groups of
objects can be clustered. A cluster is an entity that can hide its internal process objects
(and internal clusters). A complex system model is visualised as a hierarchically
levelled collection of diagrams that show clusters and process objects and their communication
connections. Figure 3.7 shows a simplified so-called Message Flow Diagram of
a Feeder station controller. Recall that we are only modelling the controller subsystem
of the machine, not the complete machine. The control system communicates with the
system parts Transporter and Feeding Unit in its environment. We call such objects in
the environment terminators and represent them by a hexagon symbol as shown. In
this case the terminators are parts that belong to the machine but not to the controller
subsystem. The controller is embedded in the rest of the system. The communication
between the entities in the diagram is visualised by various types of arrows that represent
various types of message flows (see Figure 3.8). The Feeder Controller is modelled
by a cluster. A cluster is an abstract representation. It hides its internal collaborating
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