In PLC (Programmable Logic Controller) terminology, NO and NC refer to the states of contacts in a ladder logic program. They stand for Normally Open (NO) and Normally Closed (NC) respectively, and they represent the two fundamental states of a contact in a ladder logic diagram.

1. Normally Open (NO) Contact: A normally open contact is one that is open in its normal state (i.e., when there is no actuation or energization). When the associated input or condition is true or energized, the NO contact closes, allowing current to flow through the contact. In ladder logic diagrams, NO contacts are represented by an open line or a line with a break.

2. Normally Closed (NC) Contact: A normally closed contact is one that is closed in its normal state (i.e., when there is no actuation or energization). When the associated input or condition is true or energized, the NC contact opens, interrupting the flow of current. In ladder logic diagrams, NC contacts are represented by a line that is closed or has no break.

Here's a simple example to illustrate the use of NO and NC contacts in ladder logic:

Suppose we have a start button and a stop button controlling a motor. The start button is wired to an input card of the PLC, and its associated input in the ladder logic program is represented by a NO contact. The stop button is also wired to an input card, and its associated input in the ladder logic program is represented by an NC contact.

• When the start button is pressed (true), the NO contact associated with it closes, allowing current to flow through and energize the motor relay coil, starting the motor.
• Simultaneously, the NC contact associated with the stop button opens, ensuring that even if the stop button is pressed, it cannot interrupt the flow of current to the motor relay coil.

This simple example demonstrates how NO and NC contacts are used in PLC programming to control the operation of electrical devices based on various input conditions.