Rockwell Automation 9323-S5500D A.I. SERIES MICROLOGIX 1000 AND PLC-500 SOFTW - Manual (Page 110)

Ladder Program Basics
5-5
Logical State of Rungs
During controller operation, the processor evaluates each rung, changing the state of
instructions according to the logical continuity of rungs. More specifically, input
instructions set up the conditions under which the processor will make an output
instruction true or false. These conditions are:
•
When the processor finds a continuous path of true input instructions in a rung, the
OTE output instruction will become (or remain) true. We then say that “rung
conditions are true.”
•
When the processor does not find a continuous path of true input instructions in a
rung, the OTE output instruction will become (or remain) false. We then say that
“rung conditions are false.”
Note If the actual state of an output does not agree with input conditions on a particular
rung, it may be that the output is also used elsewhere in the program. Use a cross
reference search to find any other occurrences of the output instruction. Refer to
Chapter 10 - Searching Functions for directions on conducting a search by cross
reference.
Suppose that the controller is operating and the input instructions of our rung change
state over a period of time (as determined by changing logic conditions in previous
rungs). The output instruction would then change state accordingly:
] [ ] [ / ( )
B3 B3 B3
10 11 12
XIC XIO OTE
Input Instructions Output Instruction
Inputs Output Bit Status
Time XIC XIO OTE XIC XIO OTE
t
1
(initial) False True False 0 0 0
t
2
True True Goes True 1 0 1
t
3
True False Goes False 1 1 0
t
4
False False Remains False 0 1 0