The key difference between NO (Normally Open) and NC (Normally Closed) lies in their default state when not energized:

• Normally Open (NO): Think of a light switch. In its normal state (off), the NO contact is open, meaning there's no connection and current cannot flow. When the PLC energizes the contact (like flipping the switch on), it closes, allowing current to pass.
• Normally Closed (NC): This acts like a push-button to turn off a light (already on). In its normal state, the NC contact is closed, with current flowing. When the PLC energizes it, the contact opens, interrupting the current flow.

Here's a table for a quick comparison:

In essence:

• NO = Off by default, turns on with a signal.
• NC = On by default, turns off with a signal.