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 real-world electrical circuits like relays. They refer to the default state of the contact when it's not energized.

• Normally Open (NO): An NO contact remains open (doesn't conduct electricity) in its normal state. When the PLC energizes the contact (sends a signal), it closes (conducts electricity). Imagine a light switch on a wall. When the switch is off (normal state), the circuit is open and the light is off. Flipping the switch (energizing the contact) closes the circuit and turns on the light.
• Normally Closed (NC): An NC contact acts the opposite way. In its normal state, it's closed (conducts electricity). When the PLC energizes the contact, it opens (stops conducting electricity). Going back to the light switch analogy, think of a button where pressing it turns off the light (already on). In this case, the button acts like an NC contact - normally closed, and pressing it (applying a signal) opens the circuit and cuts power.

Here's a table summarizing the difference:

NO and NC contacts are fundamental building blocks for creating logic in PLC programs. By combining them with other elements like timers and counters, you can design complex control systems for industrial automation.