Rockwell Automation 1756-RMxx ControlLogix Enhanced Redundancy System User Manual - Manual (Page 156)

156 Rockwell Automation Publication 1756-UM535D-EN-P - November 2012
Chapter 7 Program the Redundant Controller
For example, if your application uses DINTs that you use only as constants to
initialize your logic, BOOLs that you update every scan, and REALs that you
update every second, you can create a separate User-Defined Data Type for each
type of tag that is used at different points in the application. Using separate User-
Defined Data Types for each group, rather than grouping all of the tags together
in one User-Defined Data Type, helps to minimize the amount of data
transferred during the crossload.
Use DINT Tags Instead of SINT or INT Tags when Possible
We recommend that you use the DINT data type instead of the SINT or INT
data types because the controller usually works with 32-bit values (DINTs or
REALs). When processing, the controller converts SINT or INT tag values to
DINT or REAL values. When processing is complete, the controller converts the
value back to a SINT or INT value.
The controller automatically converts these data types while executing and
processing a program. No additional programming is required. However, while
this conversion process is transparent to you, it does require additional processing
time that impacts your program scan time and your switchover time.
Figure 43 - Tags Grouped into User-Defined Data Types by
Frequency of Use
Figure 43 - Tags in One User-Defined Data Type