RTU — Remote Terminal Unit

Definition​

An RTU (Remote Terminal Unit—or remote management controller in French) is an industrial electronic device designed to monitor and control equipment remotely by transmitting data to a centralized monitoring center.

Unlike a PLC, which is designed for local, high-speed control of a machine, an RTU is optimized for deployments in remote locations: low power consumption, battery or solar panel power, long-distance communication via radio, 4G/LTE, or dial-up, and rugged design for environments without regular human intervention.

RTUs are the quintessential field devices in SCADA (Supervisory Control and Data Acquisition) architectures.

Typical Uses​

RTUs are deployed in sectors that manage geographically dispersed infrastructure:

  • Water Distribution and Sanitation: pumping stations, reservoirs, wastewater treatment plants. The RTU reports water levels, flow rates, pressures, and pump statuses.
  • Power distribution networks: electrical substations, transformer stations, wind turbines, and off-grid solar farms.
  • Gas and hydrocarbon transportation: shut-off valves, compressor stations, and measurement points along pipelines.
  • Agriculture and irrigation: control of irrigation valves, weather measurements, and reservoir management.
  • Street lighting: lighting control panels, with dimming capabilities and consumption data reported to the supervisor.
  • Tunnels and engineering structures: monitoring of environmental sensors, emergency power supply, and access control.

Architecture in a SCADA System​

In a traditional SCADA architecture, the RTU operates at the field level:

Monitoring Center (MTU / SCADA)
         |
    WAN Network (4G, fiber, wireless)
         |
   RTU (site distant)
         |
  Capteurs / Actionneurs

The RTU collects data from the sensors (temperatures, pressures, levels, status), time-stamps it locally, and transmits it periodically or on an event-driven basis to the MTU (Master Terminal Unit)—the central SCADA server.

Communication Protocols​

RTUs use protocols specifically designed for remote management, which differ from standard automation protocols:

DNP3 (Distributed Network Protocol 3)​

The most widely used protocol in North America for power distribution and water management. DNP3 is robust against transmission errors, supports asynchronous communications (the RTU can report data without being queried—unsolicited response mode), and includes precise timestamping of events. It operates over RS-232, RS-485, or TCP/IP.

IEC 60870-5-101 and IEC 60870-5-104​

International standards widely used in Europe for power grid monitoring:

  • IEC 60870-5-101: serial communication (RS-232/RS-485), original version.
  • IEC 60870-5-104: TCP/IP adaptation of the 101 protocol, which has become the de facto standard for new installations.

Modbus RTU / Modbus TCP​

Although less advanced than DNP3 or IEC 60870 for remote management, Modbus is still used in many RTUs because of its simplicity and universal compatibility, particularly for communicating with field sensors.

The Difference Between RTU and PLC​

CriterionRTUPLC
Primary PurposeRemote management of remote sitesLocal control and monitoring
Power ConsumptionLow (mA to a few W)Variable (from 5 W to several hundred W)
Power SupplyBattery, solar, 12/24 V DC24 V DC or 230 V AC
Cycle timeA few seconds acceptable1 to 100 ms required
Communication4G, radio, satellite, dial-upEthernet, local fieldbus
ProtocolsDNP3, IEC 60870, ModbusModbus TCP, Profinet, Ethernet/IP, OPC-UA
Embedded logicLimited (thresholds, simple alarms)Complex (sequences, PID, motion)
Price€200 to €2,000€300 to €20,000 and up

In practice, the line between the two is blurring with modern RTUs, which offer increasingly advanced programming capabilities. Some manufacturers, such as Moxa, Advantech, and Red Lion, offer hybrid devices capable of fulfilling both roles.

Connecting to the monitoring system via 4G/LTE​

An RTU in a remote location typically connects to the monitoring center via an industrial 4G LTE modem:

  1. The RTU has a serial (RS-232 or RS-485) or Ethernet connection to the 4G modem.
  2. The modem establishes a VPN tunnel to the monitoring network to secure communications.
  3. The SCADA server periodically polls the RTU or receives its data in push mode (unsolicited).
  4. In the event of a network outage, the RTU buffers the time-stamped data locally and retransmits it upon reconnection—a critical feature for regulatory compliance in the water and energy sectors.

The Eziwan Gateway combines the functions of a 4G modem, a VPN, and a protocol gateway (Modbus-to-MQTT/cloud conversion) into a single device, simplifying deployment at existing RTU sites.

Leading Manufacturers of Industrial RTUs​

ManufacturerMain Product LinesNative ProtocolsKey Features
Schneider ElectricSCADAPack (Telvent), EasergyDNP3, IEC 104, ModbusWater/energy market in France, Unity integration
ABBRTU500, 560IEC 60870-5-101/104, DNP3, IEC 61850High-voltage and extra-high-voltage power grids
SiemensSICAM A8000, RTU3030IEC 60870, DNP3, IEC 61850Electrical substations, WinCC SCADA
GE (Vernova / Grid Solutions)UR Series, D400DNP3, IEC 104U.S. and North American markets
Red Lion ControlsFlexEdge, DA50NDNP3, Modbus, IEC 104Protocol flexibility, integrated edge computing
Wago750-8xxxModbus, IEC 60870, OPC-UAIEC 61131-3 programming, hybrid solutions
MoxaUC-8100/8200, IIoT GatewayModbus, MQTT, OPC-UAProtocol gateways, embedded Linux

IEC 61850: The Next-Generation RTU Protocol for Power Grids​

IEC 61850 is the international communication standard for electrical substations and differential protection systems. Unlike Modbus and IEC 60870, IEC 61850 is an object-oriented protocol with a self-describing data model (GOOSE messages, Sampled Values, MMS).

Although initially designed for power protection networks (very low latency, GOOSE at 4 ms), IEC 61850 is gradually being extended to remote control units (RTUs) and microgrids. For an industrial professional working with electric grid operators (ENEDIS, RTE), a basic understanding of IEC 61850 is becoming increasingly useful.

The Eziwan Gateway can communicate with IEC 61850 devices via add-on modules—contact our team for specific use cases.

Frequently Asked Questions​

Is an RTU still relevant today?​

Yes, for large-scale infrastructure (water, energy), where its remote management protocols (DNP3, IEC 60870-5-104) and robustness remain the gold standard. But for new sites, a 4G IoT gateway often meets the need at a lower cost.

What protocols does an RTU use?​

Historically, DNP3 and IEC 60870-5-101/104 have been used for remote management, along with Modbus RTU/TCP for local communication. Recent RTUs include MQTT and native IP interfaces.

How can we modernize a fleet of aging RTUs?​

Without replacing everything: a 4G gateway connects to the existing RTU via Modbus or a serial connection and transmits data to the cloud, while providing secure remote access. This allows for a phased hardware upgrade.

RTU and the Phase-Out of 2G/3G Networks: What Will Be the Impact?​

RTUs equipped with built-in GPRS modems must be migrated before the 2G/3G networks are shut down. Replacing the modem with an industrial 4G router while retaining the serial connection is the fastest way to do this.