Rockwell Automation Publication 1756-UM535D-EN-P - November 2012 157
Program the Redundant Controller Chapter 7
Use Concise Programming
Use these recommendations to create concise programming. Using concise
programming makes your program execute faster and reduces your program scan
time.
Execute an Instruction Only when Needed
We recommend that you execute instructions only when needed because each
time an instruction writes a value to a tag, the tag is crossloaded to the secondary
controller. Even if the tag values is the same, it is rewritten and is therefore
crossloaded.
Because many instructions write tag values whenever executed, strategic and
economical use of instructions is needed. Strategic programming techniques
include the following:
• Using preconditions to limit the execution of instructions
• Combining preconditions when possible
• Dividing programming into subroutines that are called only when required
• Running noncritical code every 2 or 3 scans instead of during every scan
For example, precondition an ADD instruction to run only when the controller
gets new data. As a result, the Dest_Tag is crossloaded only when the ADD
instruction produces a new value.
Figure 44 - Precondition Used with ADD Instruction
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