NOT logic in ladder circuits is typically implemented using which configuration?

• A. One normally closed switch
• B. One normally open switch
• C. Two normally closed in parallel
• D. Two normally open in series

Answer: (A)

NOT logic in ladder circuits is implemented most effectively with a normally closed contact. This works because a normally closed contact conducts when its associated input is off, allowing current to flow and energize the output. When the input becomes true (energized), the normally closed contact opens, breaking the circuit and de-energizing the output. That inversion—output on when the input is off, and off when the input is on—embodies the NOT operation directly in the rung.

Using a normally open contact would not invert; it conducts only when the input is true, so the output would follow the input instead of being its negation. Two normally closed contacts in parallel would create a more complex path that doesn’t represent a single NOT function, and two normally open contacts in series would implement an AND condition rather than a NOT.