Rockwell Automation 47C626 AutoMax Programming Executive V4.2 - Manual (Page 226)

CĆ2
Synchronizing Tasks
The implications of scan time and priority are particularly important in applications
that require tasks to share data or to be synchronized based on hardware or
software events. Priority, however, cannot be used to synchronize tasks on
different Processors. This is because each Processor controls only the scheduling
of the tasks that reside on it. Therefore, the concept of task priority is relevant only
for the Processor on which the tasks reside. Task synchronization between
AutoMax Processors and UDC modules is handled by interrupts. See the DPS
Configuration and Programming instruction manual for more information about
UDC tasks.
In addition to priority, there are other methods of synchronizing tasks that work
with both tasks on the same Processor and tasks on different Processors. In
BASIC tasks, statements such as DELAY, WAIT, SET, OPEN, and INPUT can be
used for this purpose. In Control Block tasks, the EVENT parameter in the
(required) SCAN_LOOP statement is used to synchronize tasks. PC/Ladder Logic
tasks use the EVENT coil for task synchronization.
Examples of Task Execution
The scan time required for the highest priority task on the AutoMax Processor
should be used as a guide to determine how tasks should be scheduled. It is also
necessary to factor in system overhead, e.g., communication with the personal
computer, when assigning priority and scan time. Total Processor utilization
should be kept to 80Ć85%. Processor utilization can be monitored using the
Info/Log option in the Executive software.
See the figure below for an example of task execution. Assume you have a
priority 4 PC/Ladder task that needs to be executed approximately every 12 ticks
and that takes 4 ticks to execute. Assume you also have a less critical priority 5
BASIC task that needs to be executed approximately every 17 ticks and takes 2
ticks to execute. The PC/Ladder task would start at time 0, then finish execution at
4 ticks. At this point the BASIC task would be eligible to run (its time 0).
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Priority 4 task
Priority 5 task
Time
If there are any scheduling conflicts between tasks, i.e., more than one task is
ready to run, priority takes precedence over scan time. A higher priority task that
is scheduled to run can suspend a lower priority task, regardless of whether the
lower priority task is ready to begin execution or has already begun executing.
The suspended task is allowed to execute or continue execution from where it
was suspended only after the higher priority task is finished.
Consider the following example. There are two tasks on a Processor: a priority 4
Control Block task that needs to be executed every 5 ticks and a priority 7 BASIC
task that needs to be executed every 8 ticks. Assume the Control Block task
requires 2 ticks to execute and the BASIC task requires 3 ticks to execute.