Rockwell Automation 1772-LP3 PLC - 2/30 Programmable Controller Programming and Operations Manual - Manual (Page 74)

Introduction to Programming
Chapter 4
4Ć10
Figure 4.12
Nested Branching vs. Equivalent Logic
|ą| (ą)
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B
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D
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E
|ą|
C
|ą| (ą)
A
|ą|
B
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|ą|
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C
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C
Branch
Within a
Branch
Instruction
Repeated
A. Desired Logic (Cannot be Programmed)
B. Equivalent Logic (Can be Programmed)
WARNING: While inserting a BRANCH START instruction to
an existing rung during on-line programming, the actual output
status (ON or OFF) may not be the logically expected state
of the rung. This condition exists until the BRANCH END
instruction is installed and the rung is completed.
Solution
To avoid the above condition, adhere to the following programming
technique:
1. Immediately below the rung to be changed, create a new rung with
the same conditional logic (in other words, duplicate the rung); but,
do not put the output in yet. (Figure 4.13 on the next page is an
example rung and the addition.)