In a PLC, what type of device is used specifically for high and low limit monitoring?

In a Programmable Logic Controller (PLC), the appropriate device for high and low limit monitoring is a discrete input device. Discrete input devices are used to detect specific conditions or states, often represented as on/off signals. In limit monitoring scenarios, these devices can signal when a variable, such as temperature or pressure, surpasses preset high or low thresholds.

For example, if using limit switches or proximity sensors, these devices will provide a binary signal to the PLC indicating whether the monitored condition is within acceptable limits or if action is required. This binary state (either activated or deactivated) allows the PLC to make decisions based on the inputs it receives, thus enabling effective monitoring and control of industrial processes.

While other options like analog sensors can measure varying levels continuously, they are not specifically designed to indicate high and low limits in a straightforward binary manner. Feedback mechanisms and control relays play different roles in control systems and are not typically focused on direct limit monitoring as discrete input devices are.