Rockwell Automation 9323-S5500D A.I. SERIES MICROLOGIX 1000 AND PLC-500 SOFTW - Manual (Page 116)

Ladder Program Basics
5-11
B3/10, B3/11, and B3/12 are all initially logic 0. When XIC instruction I:0/1 goes true
(e.g. because an external momentary push button closes):
•
Rung 1 is evaluated as false because XIC instruction B3/11 is false at this time.
•
Rung 2 is evaluated as true. XIC B3/11 in the branch of this rung goes true to
maintain continuity in the rung.
•
Rung 3 is evaluated as true.
•
Rung 4 is evaluated as true because XIC B3/11 has gone true. The external device
represented by OTE O:0/2 is energized.
Application: Use this program to achieve the maintained contact action of an On-Off
toggle switch using a momentary contact push button. Press for On. Press again for
Off.
The first time you press the push button (represented by address I:0/1), instruction
B3/11 is latched, energizing output O:0/2. The second time you press the push button,
instruction B3/12 unlatches instruction B3/11, de-energizing output O:0/2. Instruction
B3/10 prevents interaction between instructions B3/12 and B3/11.
Previously, we indicated that the processor executes instructions individually, rung by
rung, to the end of the program. This is called a program scan and is usually repeated
many times a second. The figure below indicates in greater detail what happens during
individual scans when an external input device (represented by I:0/1) is operated.
When the state of a bit changes during the program scan, the effects this may have in
earlier rungs of the program are not accounted for until the next scan. To point this
out, we’ve shown successive scans (1000 and 1001, 2000 and 2001, etc.).