UK What an RTU is
A remote terminal unit (RTU) is a field device that links equipment at a distant site to a SCADA system. It sits in a cabinet at a substation, pole, pumping station or wellhead, often far from the control room. It gathers readings and alarms from the equipment around it, reports them upstream over a communications link, and acts on commands sent back, so operators can see and control a network spread across a large area.
The RTU is the field end of supervisory control and data acquisition. Above it sit the SCADA servers and the operator screens, described in our comparison of HMI and SCADA. Below it sit the sensors, meters, relays and switchgear it connects to.
What an RTU does
Every RTU does four jobs, whatever the industry:
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Monitor. Reads analogue values such as voltage, current, level and pressure, status contacts such as breaker open or closed, and alarm signals from field equipment.
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Report. Sends the data to SCADA over radio, cellular, fibre or copper, either when polled or when a value changes.
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Control. Carries out remote commands, such as operating a switch, starting a pump or changing a setpoint, often with a select-before-operate check.
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Log. Stores time-stamped events locally when the link to the control room drops, then sends them when it returns, so nothing is lost.
RTU vs PLC
RTUs and PLCs both read inputs and drive outputs, and modern products overlap. The difference is mainly what each is designed around.
| Aspect | RTU | PLC |
|---|---|---|
| Main job | Telemetry: report and act on remote commands | Local control logic for a machine or process |
| Typical location | Remote, unmanned and widely spread sites | A factory, plant room or machine cabinet |
| Communications | Long-distance radio, cellular and fibre; built for slow or unreliable links | Fast local Ethernet and fieldbus |
| Protocols | DNP3, IEC 60870-5-101/104, Modbus | PROFINET, EtherNet/IP, Modbus TCP, OPC UA |
| Event handling | Time-stamped event logs, report by exception | Fast scan cycle; history usually kept elsewhere |
| Power | Often battery-backed or solar, with low consumption | Mains or a 24 V cabinet supply |
| Environment | Outdoor cabinets with wide temperature ranges | Indoor control panels |
In practice, many PLCs now speak DNP3 and many RTUs run IEC 61131-3 logic, so the choice often comes down to the protocol and the site rather than the label.
RTU communication protocols
RTUs use protocols designed for long, slow or intermittent links, where data has to be time-stamped at source and nothing can be lost:
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DNP3. Distributed Network Protocol, widely used by North American electricity and water utilities. It supports event reporting, time stamps and, in its secure version, authentication.
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IEC 60870-5-101 and -104. The common telecontrol protocols in Europe and elsewhere: -101 over serial links and -104 over TCP/IP networks.
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Modbus. A simple register-based protocol, common for reading meters, drives and instruments at the site and sometimes for the upstream link. Our FAQ on how Modbus works explains it.
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IEC 61850. The substation automation standard, increasingly used inside substations alongside DNP3 or IEC 104 to the control room.
Where RTUs are used
RTUs are found wherever an operator runs assets spread over a large area. Electricity networks use them at substations, on pole-top switches and reclosers, and at generation sites. Water companies use them at reservoirs, pumping stations and treatment works. Oil and gas operators fit them to pipelines, wellheads and compressor stations, and railways use them for traction power and trackside equipment. In each case the RTU lets one control room oversee hundreds or thousands of sites.
Because RTUs can operate switches and pumps remotely, security is now part of every design. Operators segment the SCADA network, use authenticated protocols such as DNP3 Secure Authentication, and follow IEC 62443 and IEC 62351 when hardening field devices and the links back to the control room.
RTU hardware and industrial computers
A traditional RTU is a dedicated device with fixed I/O cards. Increasingly, RTU and pole-top controller functions run on compact, fanless industrial computers, which add processing for local automation, security and data logging, and accept new software over their life. Such a computer needs serial ports for meters and relays, Ethernet, digital I/O for status contacts, a cellular or radio modem, and a wide operating temperature for unheated cabinets; our explainer on the industrial temperature range covers those ratings.
Our distribution automation case study shows that kind of system in a pole-top cabinet, and our DIN rail PCs fit the same cabinets alongside terminals and power supplies.
Quick answers on RTUs
RTUs: common questions
What is an RTU in SCADA?
The field device at each remote site. It connects local equipment to the SCADA servers, reporting data and carrying out commands from the control room.
Is an RTU the same as a PLC?
No, but they overlap. An RTU is built for telemetry over long, unreliable links; a PLC is built for fast local control. Many products now do some of both.
What is DNP3?
A SCADA protocol used by many utilities. DNP3 carries time-stamped events and commands between RTUs and control centres, and is maintained by the DNP Users Group as IEEE 1815.
What does RTU mean in HVAC?
Something unrelated. In heating and air conditioning, RTU refers to a rooftop packaged air conditioner, not a SCADA field device.
Can an industrial PC work as an RTU?
Yes. With the right I/O, communications and protocol software, a fanless industrial PC can carry out RTU functions and often adds local processing and security.
We supply fanless DIN rail and wide temperature computers for RTU and telemetry roles with the serial, I/O and cellular modules fitted as standard, and test each system before it ships. Tell us the equipment, protocols and site conditions, and we will specify the build to match.