A RTU, or Remote Terminal Unit is a device which interfaces objects in the physical world to a DCS 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. DCS is an acronym and can mean: a Digital Cellular System (DCS 1800 or GSM-1800), a mobile communications-based PCS network used outside of the U.S. a digital cross connect system a type of telecom equipment a Distributed Control System (Industrial Automation) an illness related to pressure changes... SCADA is the acronym for Supervisory Control And Data Acquisition. ...

A typical RTU has a communications interface (usually serial, Ethernet, proprietary, or any combination), a simple processor, some environmental sensors, some override switches, and a bus which it uses to communicate with devices and/or interface boards. This bus is sometimes called a device bus or a field bus. Standards include the ISO Controller Area Network (ISO 11898), MODBUS, and others. Many vendors provide proprietary buses for their equipment; examples include Allen-Bradley's Data Highway and HSQ Technology's MISERnet. Sometimes a device or field bus standard can be used to interconnect RTUs and host systems as well as field devices and RTUs. Modbus is a communications protocol based on master/slave or client/server architecture, designed by Modicon for use with its programmable logic controllers (PLCs). ...

Interface boards come in analog and digital flavors, and are typically designed for input only, output only, or both. These main types of interface boards are often abbreviated as "DI" (digital input), "AO" (analog output), and so forth. These categories are further subdivided based on the range of inputs, the amount of protection against voltage surges, and the amount of intelligence on the interface board. Sometimes an RTU or Programmable logic controller (PLC) has integral interfaces used to allow the deployment of only a RTU (without a bus and interface modules) to monitor or control just a few devices. tsx17 Télémécanique PLC PLC & input/output arrangements A programmable logic controller, PLC, or programmable controller is a small computer used for automation of real-world processes, such as control of machinery on factory assembly lines. ...

The interface boards are in turn connected to physical objects using wires. A typical application found in most SCADA implementations is to connect 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. Analog inputs are usually 24 V with a current range between 4 and 20 milliamperes (4-20 mA); the RTU or host system then translates this into the appropriate units such as gallons of water left or temperature before presenting the data to the user via the HMI or MMI. In physics, the ampere (symbol: A, often informally abbreviated to amp) is the SI base unit used to measure electrical currents. ... 4-20 ma is an analog electrical transmission standard for industrial instrumentation. ...

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. A 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. DCS is an acronym and can mean: a Digital Cellular System (DCS 1800 or GSM-1800), a mobile communications-based PCS network used outside of the U.S. a digital cross connect system a type of telecom equipment a Distributed Control System (Industrial Automation) an illness related to pressure changes... SCADA is the acronym for Supervisory Control And Data Acquisition. ...

RTUs and PLCs 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. IEC 61131-3 is a standard published in December 1993 by the IEC. It defines five PLC programming language standards: Ladder logic Sequential function chart Function block diagram Structured text Instruction list It is the third publication in the series IEC 61131. ...

External links

• Commercial RTU products:
  • SAG IDS- SCADA, RTU, SAS

See also

• Telemetry