Rockwell Automation FactoryTalk Transaction Manager User Guide - Manual (Page 80)

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FactoryTalk Transaction Manager User Guide
80
Selecting a Substitution Option
All data points must have valid values for a transaction to complete
successfully. If a data point is not retrieved and ―times out,‖ the Transaction
Control Manager service or FactoryTalk Transaction Manager service uses a
substitution value. Unscheduled data points do not have substitution
policies. If unscheduled data points are invalid, the transaction will fail.
From the Data Point Definition dialog box, select one of the listed data
points, right-click the mouse, and select Edit Selected Collection
Parameters. Choose one of the following substitution options for scheduled
data points:
 No Substitution- Specifies that the transaction fails if valid data is not
available for this data point. This occurs when the Transaction Control
Manager service or FactoryTalk Transaction Manager service times out
waiting for data or when you have bad quality data.
 Substitute Previous Value- Instructs the FactoryTalk Transaction
Manager service to use the last good value for this data point.
 Substitute Value- Allows you to specify the substitution value to use.
A substitution value of Null (default) causes the enterprise database
connector to leave the value for that column empty. A data point that has a
Null substitution policy causes a transaction to fail if the data point is used in
an expression. A Null value is not a null string or a zero; it is a value that
does not exist and, therefore, the expression evaluator cannot use it to
calculate a result.
Preventing Stale and Mismatched Data
Stale data is data that no longer matches the value in the control system.
Mismatched data refers to a set of data in which individual data values from
different times were collected and are not synchronized. Several strategies
exist to eliminate stale and mismatched data depending on the type of
transaction. For scheduled transactions, data may be read while values are
changing. For most applications, this should not be a concern (except in the