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

380 Case Study
(local) sensors and actuators (see Figure 12.2). Notice that the model does not distinguish
between the different kinds of feeders shown in Figure 12.1. This emphasises the
strive for generic modules with generic interfaces. Next to the controllers, Figure 12.2
shows a product information server. The information server produces information for
the main feeder stations and consumes information that leaves the stacker station.
12.3.2 Message Flow Diagram
The Architecture Structure Diagram of Figure 12.2 shows the structure of our conceptual
solution, but does not give any clue about behaviour and communication. A conceptual
solution of how stations communicate and exchange information can be found
by considering the requirement of personalised mail. The solution is shown in the
Message Flow Diagram of Figure 12.3. The diagram shows a cluster instance for each
Feeder
Station
accept
(PI)
accept(PI)
Merger
Station
accept(PI)
Feeder
Station
Feeder
Station
accept
(PI)
Feeder
Station
accept
(PI)
accept(PI) Product_
Information_
accept(PI)
Server
Packer
Station
accept
(PI)
Printer/
Separator
Station
accept(PI)
Figure 12.3: Message Flow Diagram of the Control System
accept
(PI)
Stacker
Station
controller with corresponding sensors and actuators. These clusters communicate by
sending each other accept ¢
messages. The idea is to create a flow of product informa-
PI£
tion objects through the controllers. Product information objects are generated by the
Product Information Server and offered to both the main Feeder Stations. The information
objects leaving the Stacker Station are accepted by the Product Information Server for further
processing3 . Product information objects are modelled by data objects of data class
PI. Each PI object contains the personal information of a corresponding physical mailing
product on the production line. The flow of PI objects is completely synchronous with
the flow of their physical counterparts, i.e. with the personalised packets. By processing
the PI objects the appropriate way, the position of each mailing product can be precisely
known, and appropriate services can be performed at the right time. To give a more
in-depth understanding of how this conceptual solution works we have to look inside
one of the station clusters.
3 The information server can for instance retransmit a PI corresponding to a damaged packet.