When is the XIO instruction true with respect to a physical input terminal?

• A. There is input voltage applied to the terminal
• B. There is no input voltage at the terminal
• C. Voltage is alternating
• D. Voltage is reversed

Answer: (B)

XIO checks for the absence of a signal at a physical input. It is true when the input terminal is not energized—there is no input voltage present. This is the opposite of an input that is energized (ON), which would make another instruction that checks for a closed or powered input true. In practical terms, if a sensor or switch applies voltage to the terminal, the input is considered ON and XIO is false; if the sensor is off or the circuit is open, there’s no voltage and XIO is true. The important idea is that XIO is about “no voltage,” not about alternating or reversed voltage. So the correct condition is that there is no input voltage at the terminal.