Saturday, 26 March 2011

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 the system and/or altering the state of connected objects based on control messages received from the system. Another term that may be used for RTU is Remote Telemetry Unit, the common usage term varies with the application area generally.

Architecture and communications

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 applications, RTUs 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. An RTU can monitor analog inputs of different types including 4 to 20 milliamperes (4–20 mA), 0 to 10 V., −2.5V to 2.5V, 1 to 5V 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. The IEC 61131 programming tool is more popular for use with PLCs, while RTUs often use proprietary programming tools

Software and logic control

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.

Comparison with other control systems

RTUs, PLCs and DCS are increasingly beginning to overlap in responsibilities, and many vendors sell RTUs with PLC-like features and vice versa. The industry has standardized on the IEC 61131-3 functional block language for creating programs to run on RTUs and PLCs, although nearly all vendors also offer proprietary alternatives and associated development environments.

In addition, some vendors now supply RTUs with comprehensive functionality pre-defined, sometimes with PLC extensions and/or interfaces for configuration. See the MultiSmart pump station manager for a water/wastewater example.

Some suppliers of RTUs have created simple Graphical User Interfaces GUI to enable customers to configure their RTUs easily. Some examples are MoxGRAF from MOX Products for their MX602 Field Controller and PC-Link from Promosys Technology for their RTU-1, RTU-3 and RTU-8.

In some applications dataloggers are used in similar applications.

A programmable automation controller (PAC) is a compact controller that combines the features and capabilities of a PC-based control system with that of a typical PLC. PACs are deployed in SCADA systems to provide RTU and PLC functions. In many electrical substation SCADA applications, "distributed RTUs" use information processors or station computers to communicate with digital protective relays, PACS, and other devices for I/O, and communicate with the SCADA master in lieu of a traditional RTU.


    * Oil and Gas remote instrumentation monitoring, (offshore platforms, onshore oilwells).
    * Networks of remote pump stations (wastewater collection, or for water supply).
    * Hydro-graphic monitoring and control, (water supply, reservoirs, sewerage systems).
    * Environmental monitoring systems (pollution, air quality, emissions monitoring).
    * Minesite monitoring applications.
    * Protection supervision and data logging of Power transmission network
    * Air traffic equipment such as navigation aids (DVOR, DME, ILS and GP)
    * Outdoor warning sirens, in both controlling them, and sending back data for verification of activation, anything broken, etc. American Signal offers this as CompuLert, and Federal Signal offers it, but isn't trademarked. Both can be setup to use DTMF or FSK for the data transport layer.