196 Rockwell Automation Publication 5069-UM002A-EN-P - January 2019
Chapter 8 Develop Control Applications
The following options rely on controlling when the main core can switch tasks.
As a result, the communication task cannot change data when the control task
is using it. Because the controller processes communications on an
independent CPU core, these methods are no longer effective in all cases.
Table 13 highlights the controller behavior.
Because the controllers have 32-bit data integrity, this only applies to data
structures larger than 32 bits. If word-level integrity is your primary concern,
the 32-bit data integrity does not impact your data use.
Good programming practice dictates the use of two unique words at the
beginning and the end of data. The controller validates the words to assure the
entire structure has data integrity.
We recommend that the handshake data is changed and the application code
validates it every transaction before the controller application code or higher-
level system reading controller data acts on it.
Table 13 - CompactLogix 5480 Controller Behavior
Application Construct
Tag Access
HMI MSG I/O Update Produce/Consume Other User Tasks Motion Planner
UID/UIE Allows Allows Allows Allows Blocks Allows
CPS Blocks Blocks Blocks Blocks Allows Allows
Periodic Tasks Allows Allows Allows Allows Allows Allows
Blocks - Helps prevent source data values from change by communications during application execution.
Allows - Communications can change source data values during application execution.
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