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Rockwell Automation Publication 2080-UM001B-EN-E - November 2011
• A User Defined Function Block (UDFB) can be executed within another
UDFB, with a limit of 5 nested UDFBs. Avoid creating UDFBs with
references to other UDFBs, as executing these UDFBs too many times may
result in a compile error.
Example of 5 nested UDFBs
Program
UDFB1
UDFB2
• Structured Text (ST) is much more efficient and easier to use than Ladder
Logic, when used for equations. if you are used to using the RSLogix500
CPT Compute instruction, ST combined with UDFB is a great
alternative.
As an example, for an Astronomical Clock Calculation, Structured Text
uses 40% less Instructions.
Display_Output LD:
Memory Usage (Code) : 3148 steps
Memory Usage (Data) : 3456 bytes
Display_Output ST:
Memory Usage (Code) : 1824 steps
Memory Usage (Data) : 3456 bytes
• You may encounter an issue with downloading and compiling a program
over a certain size. One workaround is to use arrays, especially if there are
many variables.
Running Programs with Micro800 Appendix B
UDFB3
UDFB4
UDFB5
39
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