Echelon Neuron C - Manual (Page 35)

Neuron C Programmer’s Guide 23
Letter names shown above are used for the clauses in Figure 1 and the following
narration of events. This shows how the order of execution of tasks differs from
the order the when clauses appear in a program.
At the start of this example, no event has occurred, thus no when clause event
expression is TRUE.
1 The scheduler begins with A. Since A is FALSE, its task is not executed.
2 Event C occurs and the expression C becomes TRUE.
3 The scheduler moves to B. Since B is FALSE, its task is not executed.
4 The scheduler moves to C. Since C is TRUE (item 2, above), its task is
executed.
5 A becomes TRUE.
6 The scheduler moves to D. Since D is FALSE, its task is ignored.
7 The scheduler moves back to A. Since A is TRUE (item 5, above), its task
is executed.
Figure 1. Example Scheduler Timeline
Priority When Clauses
The priority keyword can be used to designate when clauses that should be
evaluated more often than nonpriority when clauses. Priority when clauses are
evaluated in the order specified every time the scheduler runs. If any priority
when clause evaluates to TRUE, the corresponding task is executed and the
scheduler starts over at the top of the priority when clauses.
If none of the priority when clauses evaluates to TRUE, then a nonpriority when
clause is evaluated, selected in the round-robin fashion described earlier. If the
selected nonpriority when clause evaluates to TRUE, its task is executed. The
scheduler then resumes with the first priority when clause. If the nonpriority