Technical Reference Manual - InduSoft

Introduction
Executing and switching the Background Task
The Background Task executes scripts from the Math and Scheduler worksheets (for example, messages from
Alarm and Trend worksheets). In addition, the Background Task executes all Recipe and Report commands
when the Recipe or Report functions are executed during the runtime.
Although the Alarm, Math, Scheduler, and Trend tasks are not threads, you can specify or change their
Period time in the Program Settings.ini file located in the IWS program directory.
The Period default values (in milliseconds) are as follows:
[Period]
Math=100
Sched=50
Alarm=100
Trend=1000
These values mean that every 100ms, IWS generates a Period message to the Math task. Every 50ms, IWS
generates a Period message to the Scheduler task, and so on.
Caution: We strongly recommend that you do not modify the Background Task default values
unless it is absolutely necessary. Configuring any of these parameters incorrectly can cause your
entire system to malfunction (for example, CPU usage will go to 100%) and/or cause some tasks to
perform poorly.
If you must change the parameter defaults, note the values before making your changes so if a
malfunction occurs you can return to the original settings.
Keep in mind that the Background Task thread has the same priority as other threads in IWS (Drivers,
Viewer, and so forth), which means that the operating system will not execute this task continuously for more
than 20ms.
The Background Task executes the Recipe and Report worksheets when the Recipe and Report functions are
called, respectively. Because the Recipe and Report functions are synchronous, once the Background Task
starts executing the functions, it will not switch to another task (Alarm, Math, Scheduler, or Trend) until it
completely executes the functions. Executing a Recipe or Report function usually takes a few milliseconds.
The Background Task must switch between the Alarm, Math, Scheduler, and Trend tasks. When Background
Task switches to the Scheduler task, it will not switch to another task (Alarm, Math, or Trend) until all
Scheduler worksheets are executed. After executing all Scheduler worksheets, the Background Task will not
execute the Scheduler again until it receives the next Period message for the Scheduler task.
The Background Task applies the same behavior when executing the Alarm and Trend tasks — when the
Background Task switches to the Alarm or Trend task, it will not switch to another task until it handles
all pending messages. So, the Background Task will not execute the Alarm or Trend task again, until IWS
generates the next Period message for each of these tasks.
The Background Task typically executes the Alarm, Scheduler, and Trend tasks in a few milliseconds.
However, it can take longer to execute the Math task because it usually contains loops and complex scripts.
Consequently, the mechanism used to execute the Alarm, Scheduler, and Trend tasks cannot be applied to
the Math task.
The Background Task executes the Math task for no more than 10ms continuously before switching to other
task (such as the Scheduler). The Background Task cannot execute the Math task again for the next 50ms;
however, the Background Task can execute other tasks (Alarm, Recipe, Report, Scheduler, or Trend) during
this 50ms period. After the Background Task executes all of the Math worksheets, it will not begin a new scan
of the Math worksheets until IWS generates a new Period message for the Math task.
It is important to re-emphasize that this process was created to prevent 100% CPU usage all the time.
Caution: We recommend caution when using the Math function in a Scheduler worksheet or for a
screen object (such as the Command animation).
When the Scheduler task executes a Math function, no other task can be executed by the
Background Task until the Scheduler executes the entire Math worksheet called by the Math
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