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

3.6 Practical Use of Concepts 41
Therefore design of a so-called system architecture should play a role from the beginning
of a design project. Note that we are talking about the specification phase of a project
yet. In this respect the Mailing Machine line is very illustrative for a lot of design and
Product
information
server
Feeder Feeder Feeder Wrapper Separator Stacker
Figure 3.4: Mailing Machine with Network
specification problems. The plug-and-play concept leads to an architecture as illustrated
in Figure 3.4. The functionality of the system is distributed over relatively autonomous
stations that are structured into a production line. Each station gets its local controller.
Neighbour stations have plug-and-play interfaces with each other. In principle these can
be direct and generic interfaces between pairs of stations. So each line station between
two neighbour stations has at least two such interfaces.
An early design decision might provide all stations with a generic interface to a network
backbone. Figure 3.4 illustrates an architecture design imposed as a preliminary structure
of the system to be modelled. Although the initial requirement is to design autonomous
stations, it is likely that it is more efficient to provide also some centralised server
for some dedicated tasks. Figure 3.4 shows a Product Information Server which is
responsible for the distribution of product information over the system and for sending
initial configuration information to all stations. This can be information for power up
after reconfiguration of mechanical options on the machine as well for job configuration
information. The server can also gather and process information for improvement of the
quality of the processing by the machine. The requirement of personalising the product
packets also requires a conceptual solution. Figure 3.5 shows the basic solution for this
problem. The idea is to create a transport of product information objects through the
station controllers that is synchronous with the flow of the physical product packets in
the machine. The considerations so far lead to a structure of a control system that consists
of autonomous controllers. Figure 3.6 shows the controllers of stations in a machine line.
These controllers pass product information (PI) objects. Product information objects are
data objects that are transported via the plug-and-play interfaces of the stations. When
the first feeder station (left side) transports a product packet to the next feeder, their
respective controllers must transport the corresponding data object.