The simplest on/off PLC applications use _________ input devices to monitor high and low range limits of the controlled variable.

In the context of programmable logic controllers (PLCs), discrete input devices are utilized for applications requiring simple on/off control. These devices are capable of detecting distinct states, typically identified as "on" or "off," which aligns with the functionality needed to monitor high and low range limits of controlled variables.

Discrete input devices, such as limit switches, photoelectric sensors, and push buttons, respond to binary conditions. When they detect a condition that meets the preset criteria (like a certain pressure level or temperature threshold), they signal the PLC either to turn the system on or off, depending on the requirements of the control process. This is crucial in applications where only two states are relevant, such as filling a tank to a certain level or maintaining a specific temperature range.

Analog devices, on the other hand, are more suited for applications requiring variable control, as they can measure and communicate a range of values, not just binary states. Signal and continuous are terms that imply a broader scope of data handling than is necessary for the most basic on/off control tasks where discrete inputs are effective.

Thus, discrete input devices are the most fitting choice for monitoring high and low limits in applications that do not require finely tuned adjustments, making them essential for the simplest on/off PLC