Remote Terminal Unit
An RTU or Remote Terminal Unit is a microprocessor controlled electronic device which interfaces objects in the physical world to a distributed control system or SCADA system by transmitting telemetry data to the system and/or altering the state of connected objects based on control messages received from the system.
An RTU monitors the field digital and analog parameters and transmits all the data to the Central Monitoring Station. An RTU can be interfaced with the Central Station with different communication media (usually serial RS232, RS485, RS422 or Ethernet). RTU can support standard protocols (Modbus, IEC 60870-5-101/103/104, DNP3, ICCP, etc.) to interface any third party software. In some control application RTU drives high current capacity relays to digital output (or "DO") board to switch power on and off to devices in the field. The DO board switched voltage to the coil in the relay, which closes the high current contacts, which completes the power circuit to the device. An RTU can monitor analog inputs of different types including 4 to 20 milliamperes (4-20 mA), 0 to 10 V, -2.5 V to 2.5 V, 1 to 5 V etc; the RTU or host system then translates this raw data into the appropriate units such as gallons of water left or temperature before presenting the data to the user via the HMI or MMI.
RTUs differ from Programmable Logic Controllers (PLCs) in that RTUs are more suitable for wide geographical telemetry, often using wireless communications, while PLCs are more suitable for Local area control (plants, production lines, etc) where the system utilizes physical media for control.