Introduction
Tags contain process values which change while runtime is running. Value changes are handled differently at internal and external tags.
Principle
When runtime starts, the value of a tag is equal to its start value. Tag values change in runtime.
In runtime, you have the following options for changing the value of a tag:
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A value change in an external tag in the PLC.
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By input, for example, in an I/O field.
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A value assignment in a script.
Updating the value of external tags
The value of an external tag is updated as follows:
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Cyclic in operation
If you select the "Cyclic in operation" acquisition mode, the tag is updated in runtime while it is displayed in a screen or is logged. The acquisition cycle determines the update cycle for tag value updates on the HMI device. You can either choose a default acquisition cycle or define a user-specific cycle.
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On demand
If you select the "On demand" acquisition mode, the tag is not updated cyclically. The update is only carried out on demand, for example, by using the "UpdateTag" system function (
Tags.SysFct.UpdateTag (updateID)). The system function requires an update ID, which you create when you configure the tags. To update multiple tags at the same time, an update ID can be assigned to multiple tags.