Rockwell Automation FactoryTalk Transaction Manager User Guide - Manual (Page 78)

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FactoryTalk Transaction Manager User Guide
78
a data point is 512 bytes for transactions without online edits, and 4096 bytes
for transactions with online edits. If you defined a contiguous set of ASCII
registers from a PLC processor, this data point contains a complete string
(when used in a transaction). On the other hand, if you defined multiple
consecutive elements with numeric data types in the contiguous registers of
the PLC processor, you have an array. You can use the Parse() expression to
extract each element from the block data.
This feature also applies to the RSLinx Classic OPC, Generic OPC, and DDE control
connectors.
Selecting Timeout Properties
Keep the following sections in mind when selecting timeout properties.
D
ATA
V
ALID
Regardless of the trigger mechanism, once a transaction is started, the
Transaction Control Manager service or FactoryTalk Transaction Manager
service checks each required data point to determine if the value in its local
cache is still valid.
A data point value is not valid when either the data valid time has passed or
the data valid time is set to zero. The following calculation demonstrates the
requirements for validity.
Data is valid = Time Received + Data Valid value < Current
Time
Once the data point value is no longer valid, the Transaction Control
Manager service or FactoryTalk Transaction Manager service requests the
data point from a data server and starts a timer for the retrieval of that data
point. If the data point is not returned by the time specified in the data
retrieval parameter, the rules of substitution are applied.
When a data point is unscheduled, it is never requested and the transaction
fails if the data valid time has expired. Unscheduled data points do not have
substitution values. If a data point will be used as a transaction trigger and in
other transactions, set the data valid parameter to a value that is large enough