In an active high PLC input module, when is the input considered ON?

In an active high PLC (Programmable Logic Controller) input module, the input is considered ON when voltage is applied. This is because active high means that the input requires a positive voltage (typically in the range of 10-30 volts DC) to register as a logical high (or ON state).

When voltage is present at the input terminal, it allows current to flow through, signaling to the PLC that the input condition is active. This ability to detect the presence of voltage is crucial for the functioning of various control processes and automation tasks, as it enables the system to respond accordingly to changes in states of connected devices or sensors.

The other answer choices reflect states that do not provide the necessary conditions for the input to be ON. For instance, when no voltage is applied, the input would be considered OFF. Grounding the signal typically results in a logical low state. Similarly, when an input is disabled, it cannot recognize any signal, rendering it incapable of being ON. These conditions highlight the importance of a positive voltage signal to activate the input.