![]() ![]() When a coil is energized, it causes a corresponding output to turn on by changing the state of the status bit controlling that output to 1. The normally closed contact (NC) is closed when the input or output status bit controlling the contact is 0.Ĭoils represent relays that are energized when power flows to them ( Picture 1). The normally open contact (NO) is closed when the input or output status bit controlling the contact is 1. ![]() Symbolically, power flows through these contacts when they are closed. Two of the most common programming functions are the normally open (NO) contact and the normally closed (NC) contact ( Picture 1). One of the most confusing aspects of PLC programming for first-time users is the relationship between the device that controls a status bit and the programming function that uses a status bit.
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