NO and NC stand for Normally Open and Normally Closed, and they are terms used to describe the state of contacts in a Programmable Logic Controller (PLC) program or physical input devices like switches and relays.

• Normally Open (NO): An NO contact remains open (doesn't conduct electricity) in its unactivated state. When the PLC energizes the NO contact or a physical switch is pressed, the contact closes (conducts electricity). Imagine a light switch on a wall. When the switch is off, the circuit is open and the light is off. This is like an NO contact.
• Normally Closed (NC): An NC contact conducts electricity in its unactivated state. When the PLC energizes the NC contact or a physical switch is pressed, the contact opens (stops conducting electricity). Going back to the light switch analogy, think of what happens when you turn the switch on. The circuit closes and the light turns on. This is similar to an NC contact.

Here's a table summarizing the difference:

Why are NO and NC important?

NO and NC contacts are fundamental building blocks for creating control logic in PLCs. By combining them with other instructions, you can program the PLC to respond to various input signals and control outputs in a desired way. For example, you can use an NO contact to turn on a motor when a button is pressed, or use an NC contact to trigger an emergency shutdown when a safety door is opened.