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

3.6 Practical Use of Concepts 43
accept(PI)
Transporter
Feeder_Controller
productArrived mounted,
feed
move(Speed)
notMounted
encoderPulse
misFed,
doubleFed
Feeding_
Unit
Figure 3.7: Message Flow Diagram of Feeder Station Controller
accept(PI)
processes. The internal structure of the Station Controller cluster is shown in Figure 3.9.
During the modelling process we try to find and define a consistent set of collaborating
objects, that have clearly defined tasks and that encapsulate coherent data. The model
shows process objects that perform the transport of the product packet information data
objects (PI). These processes are the Product Input Handler, the Product Info Keepers and
the Product Output Handler. The Product Input Handler is responsible for the acceptance
of PI data objects from a left neighbour station. The data objects are never visualised
by graphical symbols. They are denoted textually on a message flow. The message
accept(PI) means that a PI data object will be sent in the message with name accept.
A message flow diagram shows various types of flows. For instance the accept(PI)
message requires a synchronous interaction of the left neighbour station and the
Product Input Handler. In contrast, the message flow currentPosition(Position) is a continuous
flow, which means that the sender (the Transporter Image) is continuously prepared
to send. The terminator Transporter produces productArrived messages. Notice that the
corresponding arrow head passes the symbol boundary. This symbol represent an interrupt
message flow. Such messages can interrupt the behaviour performed by a process,
and let the process switch temporarily to an alternative behaviour.
3.6.5 Formal modelling
Graphical modelling results in understanding of the system to be designed. During
modelling we play scenarios of behaviour in thought. We can consider the behaviour