A remote terminal unit is a field device that reads sensors at a remote site, controls equipment there and sends the data to a central SCADA system.
A remote terminal unit (RTU) is a small, rugged computer placed at a remote site, such as a borewell, booster pumping station, reservoir or river gauge. It reads the site's sensors, runs simple local control, and sends data to a central SCADA system or takes commands from it. An RTU has input and output modules. Analogue inputs take signals such as 4 to 20 mA from a level sensor, pressure transducer or flow meter. Digital inputs read on and off states, such as a pump running or a kiosk door open. Outputs switch pumps, motorised valves or alarms. The unit stores readings with a time stamp and sends them by radio, cellular, satellite or wired link, often using protocols such as DNP3, IEC 60870-5-104 or Modbus. Many RTUs keep logging when the link fails and send the stored data once it returns. In water, RTUs are used at sites with no staff. For example, an RTU at a remote well can read a pressure transducer every 15 minutes, stop the pump if the water level drops below a set depth to protect the pump, and report the level and pump run hours to the control room each hour. RTUs are built to cope with heat, dust and power cuts, and many run on solar panels and batteries. They differ from programmable logic controllers (PLCs), which are made for fast control inside plants, though the line between the two is now blurred. Practical issues include power at remote sites, signal coverage, sensor drift, lightning damage and cyber security, since a device that can start pumps must be protected from unauthorised commands. Clock errors can also make data from different sites hard to line up.