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

398 Case Study
Feeder
Sensors/
Actuators
Merger
Sensors/
Actuators
Feeder
Sensors/
Actuators
Feeder
Sensors/
Actuators
Feeder
Sensors/
Actuators
CAN
Packer
Sensors/
Actuators
Central Control
Station
Separator/
Printer
Sensors/
Actuators
Stacker
Sensors/
Actuators
Figure 12.10: Implementation Structure Diagram of Centralised Control System
The step from the current centralised mailing machines to the fully distributed ones
is quite large and therefore risky. To reduce the risk it would be attractive to start
off with the implementation of an ’in-between’ solution. Instead of distributing the
intelligence in a physical way, one could distribute the same intelligence in a logical
way. The resulting logically distributed control system could then be implemented in
software in a centralised sequential manner. Another implementation decision could
be to use a ’remote IO’ solution (in stead of large bundles of point-to-point wires)
for the communication with sensors and actuators. The ’remote IO’ solution could
for instance be implemented using a Controller Area Network (CAN) [CAN95]. A
graphical representation of the topology that results from the implementation decisions
is presented in the Implementation Structure Diagram of Figure 12.10. To conform to the
implementation decisions, the structure of the essential model, which was formalised as
shown in Figure 12.11, will have to be transformed. The idea is to map each terminator
instance of the Instance Structure Diagram (Figure 12.11) on a corresponding sensor
and actuator module in the Implementation Structure Diagram (Figure 12.10). The
controllers and the information server are mapped on the central control station. Using
the behaviour-preserving transformations of Chapter 10, the model of Figure 12.11 can
be transformed into a behaviour-equivalent model presented in Figure 12.12. Notice that
the Central Control Station contains a complete logical image of the physical production
line. In this way, the modularity of the physically distributed control system is preserved
in the software of the control station. Notice further that the Central Control Station
represents a distribution boundary as well as a concurrency boundary. The concurrency
boundary formalises the constraint of sequential implementation. To make the mapping
of the behaviour of the Central Control Station onto a sequential software implementation
feasible, it could be necessary to eliminate concurrency by modifying the behaviour style
of the Central Control Station.
A formal precondition for the previously described transformation to be carried out is
that the controllers (of Figure 12.11) communicate with their terminators in a weakly