Remote terminal unit
A remote terminal unit (RTU) is a microprocessor-controlled electronic device that interfaces objects in the physical world to a distributed control system or SCADA (supervisory control and data acquisition) system by transmitting telemetry data to a master system, and by using messages from the master supervisory system to control connected objects. Other terms that may be used for RTU is remote telemetry unit or remote telecontrol unit.
An RTU monitors the field digital and analog parameters and transmits data to the Central Monitoring Station. It contains setup software to connect data input streams to data output streams, define communication protocols, and troubleshoot installation problems.
An RTU may consist of one complex circuit card consisting of various sections needed to do a custom fitted function or may consist of many circuit cards including CPU or processing with communications interface(s), and one or more of the following: (AI) analog input, (DI) digital input, (DO/CO) digital or control (relay) output, or (AO) analog output card(s).
A form of power supply will be included for operation from the AC mains for various CPU, status wetting voltages and other interface cards. This may consist of AC to DC converters where operated from a station battery system.
RTUs may include a battery and charger circuitry to continue operation in event of AC power failure for critical applications where a station battery is not available.
Most RTUs incorporate an input section or input status cards to acquire two state real world information. This is usually accomplished by using an isolated voltage or current source to sense the position of a remote contact (open or closed) at the RTU site. This contact position may represent many different devices, including electrical breakers, liquid valve positions, alarm conditions, and mechanical positions of devices.
A RTU can monitor analog inputs of different types including 0-1 mA, 4–20 mA current loop, 0–10 V., ±2.5 V, ±5.0 V etc. Many RTU inputs buffer larger quantities via transducers to convert and isolate real world quantities from sensitive RTU input levels. An RTU can also receive analog data via a communication system from a master or IED (intelligent electronic device) sending data values to it.
The RTU or host system translates and scales this raw data into the appropriate units such as quantity of water left, temperature degrees, or Megawatts, before presenting the data to the user via the human–machine interface.
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