You can choose to collect data based on another tag called a Trigger Item ID and the fulfillment of the Trigger Expression.
Canary helps you accomplish this task as part of the logging process with two tools, Trigger Item ID and Trigger Expression. Here are how to use them.
- First, determine your Trigger Item ID. The Trigger Item ID references a single OPC tag whose value will determine whether all tags in the current logging group should be logged and historized.
- Next, create the Trigger Expression. The Trigger Expression is an expression where "x" represents the Trigger Item ID. When the Trigger Expression does not equal "0" or evaluates as TRUE, the tag group will begin to log data. A value of "0" or FALSE will stop logging for the entire group.
For example, imagine you have an OPC tag named "Tag001" selected as the Trigger Item ID. By entering a Trigger Expression of "x<100", when the value of "Tag001" is under 100, all the tags in the logging group will log and historize data.
If the value of "Tag001" climbs equals or climbs above 100, no data is recorded and in the historian this time frame appears as a “No Data” entry.
A variety of operators and functions may be used in the Trigger Expression. These include: