NO / NC Concept in Ladder Language

The schematic symbology for switches vary according to the switch's purpose and actuation. A normally-open switch contact is drawn in such a way as to signify an open connection, ready to close when actuated. Conversely, a normally-closed switch is drawn as a closed connection which will be opened when actuated.

Any kind of switch contact can be designed so that the contacts "close" (establish continuity) when actuated, or "open" (interrupt continuity) when actuated. For switches that have a spring-return mechanism in them, the direction that the spring returns it to with no applied force is called the normal position. Therefore, contacts that are open in this position are called normally open and contacts that are closed in this position are called normally closed.

There is a concept need to remember while using inputs in PLC programming. We can wire both NO or NC inputs to a PLC. These inputs could be pushbuttons, toggle switches, limit switches or any other input device.

Suppose we connect a NO pushbutton to I1 input of PLC. This doesn’t mean that I1 will only work as a NO. We can make I1 act as NC by programming. Also can we get the work of NO from a NC switch.

When we use I1 input of PLC, we have to take care of the state of two I1s i.e. one that is connected to I1 terminal of PLC (Hardware) and the other that we use in Ladder (Software).

Following table shows various conditions of NO/NC. You will know when will switch acts as NO and when NC. 

Concept of NO/NC

Above table shows that when the state of switch used in program is same as that of connected hardware, then that input will always work as "NO". And if there is change in state of hardware device and of ladder component, then that input will always work as "NC".