Rockwell Automation 1756-RMxx ControlLogix Enhanced Redundancy System User Manual - Manual (Page 154)

154 Rockwell Automation Publication 1756-UM535D-EN-P - November 2012
Chapter 7 Program the Redundant Controller
Manage Tags for Efficient Crossloads
To program for more efficient crossloads of data and to reduce the amount of
time required for a crossload to execute, manage your data tags as recommended
in these sections.
Delete Unused Tags
Deleting unused tags reduces the size of the tag database. A smaller database takes
less time to crossload.
Use Arrays and User-Defined Data Types
If you use arrays and User-Defined Data Types, the tags use smaller 4-byte (32-
bit) words for all of the data in the type or array. If you create an individual tag,
the controller reserves 4 bytes (32 bits) of memory even if the tag uses only 1 bit.
Arrays and User-Defined Data Types help conserve the most memory with
BOOL tags. However, we also recommend you use them for your SINT, INT,
DINT, REAL, COUNTER, and TIMER tags.
Figure 40 - Example Savings with the Use of an Array
For more information about working with arrays, User-Defined Data Types, and
alias tags, see the Logix5000 Controllers I/O and Tag Data Programming
Manual, publication 1756-PM004 .
4 bytes of data to crossload.
12 bytes of data to crossload (4
bytes for each tag).
TIP If you have already created individual tags and programming that uses
those tags, consider changing the individual tags to alias tags that
reference the elements in an array.
If you choose to do this, your programming can still reference the individual
tag names, but the crossload transfers the base array.