Allen-Bradley 1756-CNB User Manual page 80

80 Configure and Program the Controller
Action
7. Execute code only when you need it.
Publication 1756-UM523F-EN-P - December 2006
Details
The controller crossloads a tag anytime an instruction writes a value to the tag. This
happens even if the value stays the same.
• Many instructions write a value whenever they run. For example, instructions such as
OTL, OTU, and many instructions with Destination operands write a value each time
the rung-condition-in is true.
• Whenever an instruction writes a value, the controller marks the value for the next
crossload. This occurs even if the instruction wrote the same value that was previously
in the tag.
If you execute an instruction only when you need to, you reduce the amount of crossload
data. This reduces scan time. To limit the execution of an instruction:
• make a rung false when you do not need to execute its instructions.
• divide your logic into subroutines and call each subroutine only when needed.
• run noncritical code every few scans instead of every scan.
This is better
The ADD instruction runs only when the controller gets new data (New_Data = on). And
Dest_Tag crossloads only when the ADD instruction produces a new value.
Than this
The ADD instruction writes the sum of Tag_1 + Tag_2 to Dest_Tag each time the rung
executes. The controller crossloads Dest_Tag every scan, even if Tag_1 and Tag_2 stay the
same.