Each Logic program has an Execution Method setting that you can use to define whether the program is executed as a response to its inputs or on a timed basis.
The Execution Method setting provides two options:
- On Input Processed—The program is executed whenever one of its inputs is processed. This means that every time the ClearSCADA server receives a value for one of the inputs, it will execute the program (even if the value has not changed since the last time the program was executed).
ClearSCADA will only execute the program if the inputs are deemed to be valid. The following inputs are valid:
- %I — but only if the program does not use the NOCACHE attribute, and the tag for the variable references a field, method, aggregate or historic tag
- %M — but only if the program has no assignments (does not write to the variable), does not use the NOCACHE attribute, and the tag for the variable references a field, method, aggregate or historic tag.
The following inputs are invalid:
- %O
- Inputs using NOCACHE
- Inputs that reference a register or status tag
- %M inputs that have assignments.
If the program does contain a valid input, it will not execute and a message will be displayed.
The On Input Processed setting should not be confused with the Input Change Detection setting, which causes the program to be executed whenever ClearSCADA detects that the value of one of the inputs has changed since the previous execution. For more information about the Input Change Detection setting, see Define whether a Logic Program Executes when the Value of an Input Changes.
- On Interval—The program is executed regularly at timed intervals. You need to define the timed intervals in the Interval and Interval Offset fields (see Define the Interval Settings for a Logic Program).
To change the Execution Method of a Logic program, display the Logic Program Form, select the Logic tab, then choose the appropriate setting from the Execution Method combo box.
Further Information
Display the Logic Program Form: see Configure an ST Program, Ladder Diagram, SFC or FBD.