Limit value
The limit value trigger initiates a transfer when a limit value (OPC item) is exceeded or not reached.
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The individual properties are:
| Property | Description |
|---|---|
| Connection | OPC connection through which the relevant data point is accessed. |
| Data point | OPC data point whose value triggers the transfer when it is exceeded or not reached. Use the "..." button to open the tag browser (see below) for interactive selection of an item. For OPC UA connections, the node ID is managed in the background. |
| Limit value | Absolute value at which the trigger is activated if exceeded or not reached. |
| Trigger when data point | Specifies whether the trigger is activated when the limit value is exceeded or not reached. |
| Hysteresis |
Hysteresis:
Once the trigger has been triggered, depending on the application context, it may be desirable for the trigger to only trigger again once the value of the OPC item has moved away from the limit value. In this case, set the necessary difference from the limit value at which the trigger is "activated" again. All specifications refer to the limit value.
Example: The limit value is 20 ??C, and the trigger is activated when this value is exceeded. Now the temperature fluctuates around the 20-degree mark, which would trigger the trigger several times. However, only the first exceedance is relevant for recording. Therefore, the hysteresis value is set to "Reset trigger from 18 degrees absolute value." The trigger is activated once when the 20-degree mark is exceeded. It will not trigger again until the temperature has fallen below 18 degrees and then risen above 20 degrees again.
Hysteresis options:
| Property | Description |
|---|---|
| Reset trigger from % difference | The trigger will only trigger again when the value difference has exceeded the specified percentage. |
| Reset trigger from absolute difference | The trigger will only trigger again when the value difference exceeds the specified absolute difference. |
| Reset trigger from absolute value | The trigger will only fire again when the value difference corresponds to the specified value. |
Tip: If you want the connection to be permanently triggered in the "Off-Limit" state, set the absolute hysteresis value to the maximum value that the element can assume. This way, it will trigger in the OPC router cycle time interval as long as it is in the off-limit state. Combine with a data change trigger linked with AND to trigger only data changes above the threshold. In the example above, the temperature will not exceed 100 degrees. With threshold = 20 and trigger reset from 100 absolute value, the trigger would fire whenever the temperature is between 20 and 100 degrees, at the rhythm of the router's cycle time (settings). You could restrict this with an AND trigger for data changes to record only data changes above the threshold value.
Data change and threshold triggers for analog values
You use data change and threshold triggers when you want to transfer a single analog value. The trigger item and transfer item are identical. The transfer starts immediately after the trigger condition occurs. If the OPC transfer object contains other items to be transferred, it is therefore possible that values may be transferred that do not belong together in terms of content.