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How Does an Expression Trigger Work?


Last Update: 2018/11/12

An Expression Trigger allows users to specify a condition at which logging will start and stop. For example, users could specify that a Boolean/Discrete Tag in the server must be in an On/True state in order for the trigger to be active. Once it goes to an Off/False state, the trigger will become inactive. Users could also specify that the tag value must be greater than or equal to a value in order to start logging. Once it goes below that value, logging will stop. Both Static Interval and Data Change logging starts when the trigger is active. For example, if a Static Interval is set to 1 minute, that minute will be calculated from the time that the trigger became active. As such, the first record would be logged 1 minute after starting. Data Changes are also determined from the start of the trigger activation. The following new trigger functions are available for Expression Triggers:

  • Log once when trigger start condition is met: This function logs all the current values for the items in the Log Group once when the trigger starts. In KEPServerEX V5.5 and older versions of the server, this was referred to as a Start Snapshot.
  • Log once when trigger stop condition is met: This function logs all the current values for the items in the Log Group once when the trigger stops. In KEPServerEX V5.5 and older versions of the server, this was referred to as a Stop Snapshot.

Note: KEPServerEX V5.5 and older versions of the server require that the Static Interval or Data Change method be selected along with a snapshot condition. KEPServerEX V5.6 allows users to only select a snapshot condition (leaving the Static Interval and Data Change methods unchecked).

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