NAND is implemented in ladder logic by which configuration?

• A. Two XIC In Series
• B. Two XIO In Parallel De-energize If Both Engaged
• C. Two XIC In Parallel Energized If Any Closed
• D. Two XIO In Series

Answer: (B)

The key idea is implementing a NAND function in ladder logic by using normally closed contacts and wiring them in parallel. Two XIO contacts (examine if open) invert each input: each contact closes when its associated input is off, and opens when the input is energized. Placing them in parallel creates a conducting path as long as at least one input is off, so the output coil is energized. Only when both inputs are energized do both XIO contacts open, breaking the path and causing the output to de-energize. This yields the NAND behavior: the output is on in all cases except when both inputs are on.

Other configurations don’t match NAND: two XIC in series would require both inputs on to energize (an AND behavior); two XIC in parallel would energize if any input is on (an OR behavior); two XIO in series would energize only if both inputs are off (not NAND).