Remote terminal unit

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. Counter inputs are optional.

An 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 the quantity of water left, temperature degrees, or Megawatts, before presenting the data to the user via the human–machine interface.

RTUs may drive high current capacity relays to a digital output (or "DO") board to switch power on and off to devices in the field. The DO board switches voltage to the coil in the relay, which closes the high current contacts, which completes the power circuit to the device.

RTU outputs may also consist of driving a sensitive logic input on an electronic PLC, or other electronic device using a sensitive 5 V input.

While not as commonly used, analog outputs may be included to control devices that require varying quantities, such as graphic recording instruments (strip charts). Summed or processed data quantities may be generated in a master SCADA system and output for display locally or remotely, wherever needed.

Modern RTUs are usually capable of executing simple programs autonomously without involving the host computers of the DCS or SCADA system to simplify deployment and to provide redundancy for safety reasons. An RTU in a modern water management system will typically have code to modify its behavior when physical override switches on the RTU are toggled during maintenance by maintenance personnel. This is done for safety reasons; a miscommunication between the system operators and the maintenance personnel could cause system operators to mistakenly enable power to a water pump when it is being replaced, for example.

Maintenance personnel should have any equipment they are working on disconnected from power and locked to prevent damage and/or injury.

An RTU may be interfaced to multiple master stations and IEDs (Intelligent Electronic Devices) with different communication protocols (usually serial (RS-232, RS-485, RS-422) or Ethernet). An RTU may support standard protocols (Modbus, IEC 60870-5-101/103/104, DNP3, IEC 60870-6-ICCP, IEC 61850 etc.) to interface any third party software.

Data transfer may be initiated from either end using various techniques to ensure synchronization with minimal data traffic. The master may poll its subordinate unit (Master to RTU or RTU to IED) for changes of data on a periodic basis. Analog value changes will usually be reported only on changes outside a set limit from the last transmitted value. Digital (status) values observe a similar technique and only transmit groups (bytes) when one included point (bit) changes. Another method used is where a subordinate unit initiates an update of data upon a predetermined change in analog or digital data. Complete data transmission must be performed periodically, with either method, to ensure full synchronization and eliminate stale data. Most communication protocols support both methods, programmable by the installer.

Multiple RTUs or IEDs may share a communications line, in a multi-drop scheme, as units are addressed uniquely and only respond to their own polls and commands.

IED communications transfer data between the RTU and an IED (Intelligent Electronic Devices) . This can eliminate the need for many hardware status inputs, analog inputs, and relay outputs in the RTU. Communications are accomplished by copper or fibre optics lines.

Master communications usually occur between an RTU and a larger control system or a data collection system (incorporated into a larger system). Data may be moved using a copper, fibre optic or radio frequency communication system.