How to
Configuring a Change of Value Trend Log
You configure the properties of a change of value trend log to meet the unique needs of your site.
The change of value trend log records the variable each time the difference between the current value and the last log value exceeds the delta. Use the change of value trend log to, for example, record a variable that has an unequal oscillation.
The change of value trend log is limited to 10 records per second.
Do not set the delta too small since this can quickly fill up the trend log.
For example, in the figure below, the change of value trend log records a variable every time the variable exceeds or falls below the delta of 3. The records are printed in a trend chart. Notice that the change of value trend log records the values without any time perspective between the records.
The change of value trend log, which is not a variable triggered trend log, has the function to start log records according to a given start condition. For more information, see Trend Log Activation .
You can configure the maximum log interval in a change of value trend log. The maximum log interval is the maximum time that can elapse between two recordings regardless of whether the value has changed or not. The maximum log interval is calculated from the last actual recording. When the maximum log interval is configured, it bypasses the delta and the recording is triggered when the configured time period has elapsed.
In WorkStation, in the System Tree pane, click the change of value trend log you want to configure.
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Edit the properties:
Table: Change of Value Trend Log Properties - Basic Tab
Property
Description
Status
Displays the status of the trend log.
Enabled
Displays True if the trend log is enabled.
Logged variable
Enter the variable that you want to log.
Delta
Select the minimum value change that triggers a new record.
Activation variable
Enter the variable that makes the trend log start recording values when the variable is True .
For more information, see Trend Log Activation .
Activation time
Specify the time when the trend log is to start recording values.
For more information, see Trend Log Activation .
Maximum log interval
Select the maximum time that can pass between two recordings regardless if the delta value has been exceeded or not.
Log size
Select the number of records to store before old records are overwritten, including internal records generated by the trend log.
For more information, see Log Record Storage .
Clear when enabled
Select True to clear the trend log when it is enabled.
Click the Save button
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Change of Value Trend Logs
Change of Value Trend Log Properties
Configure Dialog Box – Configuration Tab
Trend Log Activation
Configure Dialog Box – Operation Tab
Log Record Storage
Trend Log Does Not Start Record Due to the Start Condition
Missing Trend Log Records