A Simple Ladder Diagram with NO and NC Contacts

The below figure illustrates an example of a simple ladder program with the NO and NC contacts driving an output rung.

Ladder Diagram with NO and NC Contacts

Ladder-diagram-with-NO-and-NC-contacts

With the limit switch LS0 open, when SP0 is pushed the reference input SP0 (I: 1/0) is closed, the output coil PL1 (O: 3/0) is energized closing all (O: 3/0) contacts and the pilot light PL1 will be lit.

Control contact (O: 3/0) in parallel with contact I: 1/0 then closes sealing in the output coil PL1 even if the start button is released.

Since output PL1 (O: 3/0) is ON, the normally open contact (O: 3/0) in the second rung will close, turning the internal output coil INT_OTE_1 (B3/1) ON.

The NC contacts (O: 3/0) in the third rung will open because the examine for an OFF condition is not true (reference output is ON), therefore turning internal output INT_OTE_2 (B3/2) OFF.

Note that outputs B3/1 and B3/2 will not turn a real output device ON because these internal bits are not mapped to the output device.

When the limit switch LS0 is closed the NC contact LS0 (I: 1/1) in the first rung opens, the output O: 3/0 is de-energized, and the pilot light PL1 is turned off.

The normally open contact (O :3/0) in the second rung will open, turning the internal output coil INT_OTE_1 (B3/1) OFF.

The NC contact in the third rung is closed turning the internal output coil (INT_OTE_2), B3/2 ON.

Author: Dr. Hadi Saadat

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