Figure 1:Start of concurrent processes
A start of two
concurrent processes is shown in the figure above. The double line after the
transition indicates the concurrent start of the processes A and B. Observer
that the transition ‚Start processes
A&B‘ is preceding the double
line, indicating that that condition has to be true before the 2 processes can
be started.
Example:
Figure 2: Example start of concurrent processes
After the start
¨bang’ which is the transition condition the 3 athletes start running the race
independently.
Figure 3: Synchronization of concurrent processes
The two processes
A and B are synchronized at the double line. The SFC-switching rules,
investigated in the sequel, enforce that all the steps preceding the double
line have to be marked, before the go on transition can fire.
Observe: the
transition follows the synchronization line.
Example:
Figure 4: Synchronization example
When three
persons drive with one car, they have to be ‘synchronized’, i.e. they have to
be at the same time at the same place before they can start their journey.
Obviously, it is easier to wait in a comfortable chair. Similarly, it is
recommended to use dedicated waiting steps ahead of synchronization in order to
solve all waiting functionality in a neat way.
In Lesson 2, the switching rules were introduced. In this section, the switching rules
are reinvestigated with respect to process synchronization. A transition is
called ‚enabled‘, if all directly preceding steps are marked. When a synchronization
bar is ahead of the transition, all the steps connected to the bar have to be
marked. This is shown in the next figure.
Figure 5: Switching Rules at synchronization