Digital Indicators, Scanners, and On-Off Controllers are essential devices used in industrial automation and control systems to monitor, display, and manage process variables like temperature, pressure, and flow.

1. Digital Indicators: These are used to display the readings from sensors or transmitters, converting analog signals into clear digital values. They offer high accuracy and are often used in applications where precise monitoring is crucial. They may include features like alarms, data logging, and remote display options.

2. Scanners: Scanners are used to sequentially monitor and display multiple inputs from various sensors. They are ideal for applications where there are multiple points needing constant observation, like temperature or pressure across several locations. Scanners can automate the process of switching between different input channels and provide real-time monitoring.

3. On-Off Controllers: These controllers are used to maintain a specific process variable within a set range by turning equipment on or off. For example, they may control a heating system by switching the heater on when the temperature falls below a setpoint and off when it exceeds the desired temperature. On-Off controllers are simple, reliable, and cost-effective for maintaining basic process control.

These devices work together to provide efficient, automated process monitoring and control in various industries, from manufacturing to energy production.