Rockwell Automation 57C620 AutoMax Programming Executive Version 3.8 - Manual (Page 237)

CĆ1 AutoMax Task Execution
Appendix C
AutoMax Task Execution
In AutoMax applications that incorporate multiĆtasking, task execution is
determined by eligibility to run. A task's eligibility, in turn, is determined by its
priority, scan time, and any links to other tasks built into the task. This appendix
describes task execution for AutoMax application tasks. Refer to the appropriate
Configuration and Programming instruction manual for a description of the task
execution for UDC tasks.
Priority
Task priority refers to the relative importance of a task in the application. Tasks
whose execution is more critical to the operation of the controlled machinery
should be assigned higher priority. Priority can range from 4 (highest) to 11
(lowest). Priority is specified by the programmer when adding a task to the rack.
Scan Time
Scan time refers to how often the task is scheduled to be executed. It is usually
measured in ticks. You can assign a tick rate for each AutoMax Processor. The
tick rate can range from 0.5 ms to 10.0 ms. The default tick rate is 5.5 ms. See
section 6.1 of this manual for more information. For example, an AutoMax task
with a 20 tick scan time (using the default tick rate) means that the task is
scheduled to start every 110ms.
Scan time is specified differently for PC/Ladder Logic, Control Block, and BASIC
tasks. In each case, before setting the scan time, it is important to know
approximately how long the task takes to execute. For example, a task that takes
20ms to execute cannot be assigned a scan time of 2 ticks because a 20ms task
cannot be started every 11ms. This situation would result in an overlap error
(error code 14 would be displayed on the Processor and all tasks in the rack
would be stopped).
Scan time for PC/Ladder Logic tasks is set using the PC Editor in the Executive
software. The F3 key, followed by I, allows the programmer to enter the scan time.
The screen display will indicate the approximate execution time of the task,
determined by the Executive software. The AutoMax Ladder Logic language
instruction manual contains execution time estimates for each type of Ladder
Logic operation.
For AutoMax Control Block tasks, the scan time is specified in ticks using the
SCAN_LOOP statement. The approximate execution time of the task can be
calculated using the execution time estimates found in the AutoMax Control Block
language instruction manual (JĆ3676).
The scan time for BASIC tasks is usually specified in the START EVERY
statement. The START EVERY statement, which is optional, allows the programer
to use seconds, minutes, and hours, as well as ticks, as the time unit. Note that
there is no simple method of estimating execution time for BASIC tasks ahead of
time. One method of estimating execution time for a specific BASIC task is to
include a statement that turns on a DC output at the beginning of the task (after
the START EVERY statement, if used) and turns it off at the very end of the task.
Then the task is put into run with no other tasks running, and an oscilloscope is
used to measure the time that the output is on.