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.

An electromechanical relay converts electrical energy into motion to open or close electrical contacts. When current flows through its coil, a magnetic field is produced that pulls a metal armature, causing the contacts to move and switch the load circuit. This combination of electrical input and mechanical movement is what defines an electromechanical relay. Because it relies on a magnetic field created by electricity to drive the mechanical action, it’s not correct to say it uses only mechanical energy. And since it changes the state of a circuit by moving contacts, it can switch currents in the load. It also has moving parts (the armature and contacts), so describing it as having no moving parts would be inaccurate. For contrast, solid-state relays switch without moving parts, but that’s a different type.

In digital logic, an OR gate outputs a high whenever any of its inputs is high. For two inputs, the output is low only when both inputs are low; if one input is high (or if both are high), the output goes high. This shows the inclusive nature of OR: the presence of at least one high signal is enough to produce a high output. In practical terms, if any one of several signals indicates a condition, the OR gate can trigger the next stage. This differs from AND, which needs all inputs high to produce a high, and from equality-based ideas, which look at whether inputs match rather than whether any one input is high.

The test is about matching how the sensor outputs signal to how the PLC input expects to receive it so a complete current path is created. A current sourcing sensor pushes current out into the circuit. To make that current flow into the PLC input and return to ground, the PLC input must be able to sink current. That’s why a current sinking input module is the correct match—the input provides a path to ground for the sensor’s sourced current. If you used a current sourcing input instead, both sides would be sources and no proper current flow would occur, so the signal wouldn’t be detected.