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

382 Case Study
tends the functionality of the terminator with a number of powerful services. The
Transporter Image translates the encoderPulse into a unity of Position. A Position is a data
object recording the relative position of a physical mailing product. The difference between
two Positions is a measure for the distance of product transport. This difference
is represented by a data object of class PositionDelta 4 .
If a physical mailing product is about to enter the feeder station, the
Product Input Handler is being sent an accept(PI) message by its left neighbour station.
A PI 5 is a data object that contains the required services of the corresponding mailing
product. It also knows whether the required services have yet been performed
to a correct result or not. If the feeder station can receive the mailing product, the
Product Input Handler accepts the message accept(PI). If the rendezvous has occurred,
the neighbour station knows that the feeder is willing to receive the corresponding
physical mailing product, and the feeder station knows that a new product will enter
the station soon. After reception of the accept(PI) message the Product Input Handler
waits until it receives the message productArrived from the Transporter Image. Then it immediately
reads the current product Position 6 and stores the corresponding PI in one of
the free Product Info Keepers. The multiple of Product Info Keepers is found naturally by
realising that each PI entering the control station must be stored somewhere for further
processing.
Each instance of the multiple of Product Info Keepers has an identifier represented by
a data object of class PIKeeperId. Whenever a Product Info Keeper is free, it sends its
identifier to the Product Input Handler. After the PI object is stored, it remains inside the
keeper until the corresponding mailing product leaves the feeder station again. After the
PI object is stored, the Product Input Handler sends a message schedule(InitialPosition) to
the Service Scheduler. Data class InitialPosition is a specialisation of class Position (see also
Subsection 12.3.7). An InitialPosition records the relative entrance position 7 of a mailing
product. After the message schedule(InitialPosition) is sent, the Product Input Handler
calculates the Position at which a new product can safely enter the feeder station. Then
it requests the Transporter Image to get woken up at this Position, after which it starts
waiting for message wakeUp to arrive. Until then no new PI is accepted.
The Service Scheduler takes care that all matching required services, that are recorded in
the Product Info Keeper, are actually being provided. Within the Product Flow scenario
only one service is offered. This service deals with the handing over of the mailing
product to the right-sided neighbour station. The hand over service is triggered when
the Product Output Handler receives message handOver(InitialPosition). Upon reception
of this message the Product Output Handler inspects the InitialPosition of the mailing
product and calculates two other Positions. Then it requests the Transporter Image to
get woken up at these Positions and consequently it starts waiting for the arrival of
4This data class is not shown as parameter type of any message in Figure 12.4.
5PI is an abbreviation of Product Info.
6Notice that this position can always be read immediately, since the flow currentPosition(Position) is
continuous.
7This position is relative to some fixed point of a transport belt.