Remote Terminal Units (RTUs) are widely used to collect field data, monitor equipment, and support remote control in industrial systems. To exchange information with controllers, sensors, and supervisory platforms, they rely on communication protocols.
But RTU communication does not follow one universal protocol. Depending on the industry and system architecture, an RTU may use Modbus, DNP3, IEC 60870-5, or other communication standards.
So, what are the main RTU communication protocols, and how do they differ? Let’s look at how they work, where they are used, and what to consider when connecting devices across different networks.
How RTU Communication Protocols Handle Data
An RTU typically collects information from field devices, such as meters, sensors, and controllers. It then sends this data to a control center or supervisory system, where operators can monitor conditions and issue commands.
The communication protocol defines how devices exchange this information. It specifies how messages are structured, how data is identified, and how communication is managed.
Polling and Event-Based Reporting
RTU systems may use different methods to report field data.
In a polling-based system, a master or supervisory device requests information from an RTU. The RTU responds with the requested data. Modbus RTU commonly follows this request-response model.
Some protocols also support event-based reporting. Instead of waiting for every regular request, an RTU can report important changes or events when they occur. DNP3, for example, supports unsolicited responses, allowing an outstation to send event information without waiting for a specific poll.
The exact behavior depends on the protocol and system configuration.
Serial and IP-Based Communication
RTUs can communicate through serial interfaces or Ethernet-based networks.
Serial communication is common in established industrial installations. RS-232 and RS-485 are often used to connect RTUs with meters, controllers, and other field devices. Modbus RTU is a familiar example of a serial communication protocol.
Ethernet and IP networks allow RTUs to communicate over local networks or wider-area connections. Protocols such as Modbus TCP and IEC 60870-5-104 are designed for communication over TCP/IP networks.
It is important to distinguish the protocol from the communication medium. RS-485 defines electrical signaling, while Modbus RTU defines how data is exchanged over a serial connection.
RTU Protocols Across Different Industries
Different industries have different requirements for data collection, remote monitoring, control, and network communication. As a result, RTU-based systems may use different protocols.
Power Systems and Substations
Power systems often require reliable communication between remote substations, control centers, protection equipment, and monitoring devices.
Common protocols and standards include:
DNP3
DNP3 is used in electric utility and other supervisory control applications. It supports structured data reporting, time-stamped events, and unsolicited reporting under suitable configurations. These features are useful when operators need to monitor changing field conditions across remote sites.
IEC 60870-5-101
IEC 60870-5-101 is a telecontrol protocol designed for communication between control systems and remote stations. It is commonly associated with serial communication in power automation.
IEC 60870-5-104
IEC 60870-5-104 extends the IEC 60870-5-101 application layer for communication over standard TCP/IP networks. It is often used when remote stations need to communicate with control centers over Ethernet or IP-based infrastructure.
IEC 61850
IEC 61850 is a broader communication standard for power utility automation, particularly substation automation. It defines data models and communication services for interoperability among substation devices.
IEC 61850 is not simply another serial RTU protocol. It is commonly used in systems involving intelligent electronic devices (IEDs), substation controllers, and other power automation equipment.
Industrial Automation
Industrial automation systems often combine PLCs, remote I/O, meters, and other field devices. Protocol requirements depend on the equipment and control architecture.
Modbus RTU
Modbus RTU is a widely used serial protocol in industrial automation. It commonly operates over RS-485 or RS-232 and uses a master-slave communication model, now often described as client-server terminology.
Its relatively simple message structure makes it suitable for communication with devices such as PLCs, temperature controllers, and energy meters.
Modbus TCP
Modbus TCP carries Modbus messages over TCP/IP networks. It is commonly used for communication between Ethernet-connected devices, including PLCs, HMIs, and supervisory systems.
Modbus RTU and Modbus TCP share the Modbus application protocol, but they use different communication formats and transport mechanisms.
PROFIBUS and PROFINET
PROFIBUS is a fieldbus technology used in industrial automation, including communication between controllers and distributed field devices.
PROFINET is an Industrial Ethernet standard that supports communication between automation components over Ethernet networks.
Although both are associated with industrial automation, they are distinct technologies and should not be treated as interchangeable protocols.
Building Automation and Industrial IoT
Building management and connected industrial systems may involve RTUs, controllers, gateways, and supervisory platforms that use different communication technologies.
BACnet
BACnet is a communication protocol designed for building automation and control networks. It supports the exchange of information among systems such as HVAC, lighting, and other building services.
BACnet can operate over different network technologies, including BACnet/IP and BACnet MS/TP.
MQTT
MQTT is a lightweight publish-subscribe messaging protocol commonly used in IoT applications. It allows devices or applications to publish data to a broker, which distributes messages to subscribed clients.
MQTT is not a traditional RTU telecontrol protocol. However, it can be used in RTU-based systems when field data needs to be forwarded to an IoT platform, cloud service, or remote monitoring application.
What Makes RTU Communication Protocols Different?
Although these protocols may serve similar monitoring or control purposes, they differ in several important ways.
| Protocol | Communication model or network | Typical applications | Key characteristics |
| Modbus RTU | Serial, commonly RS-485 or RS-232 | Industrial automation, meters, controllers | Simple request-response communication |
| Modbus TCP | Ethernet/TCP/IP | PLCs, HMIs, SCADA networks | Modbus messaging over TCP/IP |
| DNP3 | Serial or IP-based implementations | Utility monitoring and control | Supports event reporting and time-stamped data |
| IEC 60870-5-101 | Serial telecontrol | Power system remote stations | Communication between control systems and remote stations |
| IEC 60870-5-104 | TCP/IP | Power system telecontrol over IP networks | IEC 60870-5-101 application layer over TCP/IP |
| IEC 61850 | Ethernet-based substation communication | Substation automation | Standardized data models and communication services |
| PROFIBUS | Fieldbus | Industrial automation | Communication between controllers and field devices |
| PROFINET | Industrial Ethernet | Industrial automation | Ethernet-based automation communication |
| BACnet | Various network options | Building automation | Designed for building control and monitoring |
| MQTT | TCP/IP, publish-subscribe | IoT and remote monitoring | Message distribution through a broker |
The table provides a general overview. Actual capabilities depend on the protocol profile, device implementation, and system configuration.
Communication Model
Some protocols mainly use request-response communication, while others support event reporting or publish-subscribe messaging.
For example, Modbus RTU typically relies on a client or master requesting data from a server or slave. DNP3 supports event reporting, while MQTT uses a broker-based publish-subscribe model.
These differences affect how data is requested, delivered, and handled during communication interruptions.
Network and Transport
The physical network and transport method also vary.
Modbus RTU commonly operates over serial links, while Modbus TCP and IEC 60870-5-104 use TCP/IP. PROFIBUS and PROFINET belong to different industrial networking technologies, even though both are used in automation.
A device supporting one network type may need additional equipment or a different interface to communicate with devices on another.
Data Representation
Protocols also differ in how they identify and represent information.
Modbus uses registers and coils. DNP3 uses data objects and supports event information. IEC 61850 uses standardized data models designed for power utility automation.
These differences matter when one system needs to interpret data originating from another protocol. Matching the network connection alone does not guarantee that the receiving system can understand the data.
Can Different RTU Protocols Work Together?
Yes, but interoperability depends on the protocols, device capabilities, and the type of data being exchanged.
Two devices may use different protocols and still communicate through a suitable intermediary. However, connecting their network interfaces is only one part of the process. The data must also be mapped into a format that the receiving device or application can understand.
Modbus RTU and Modbus TCP Compatibility
Modbus RTU and Modbus TCP share the Modbus application protocol, but they use different communication formats.
A Modbus RTU device connected through RS-485 cannot necessarily communicate directly with a Modbus TCP device over Ethernet. A suitable converter or gateway may be needed to bridge the serial and Ethernet networks.
Depending on the equipment, this can allow a PLC, HMI, or SCADA system to access data from existing serial Modbus devices without replacing them.
DNP3, IEC 60870, and IEC 61850 Interoperability
Power automation systems may contain equipment using DNP3, IEC 60870-5-101, IEC 60870-5-104, or IEC 61850.
These standards are not directly interchangeable. Their data models, communication services, and supported functions differ.
When information needs to move between them, an appropriate protocol gateway or system-level integration solution may be required. The supported conversion functions must be checked carefully, especially when the application involves alarms, time-stamped events, control commands, or power-system data models.
Connecting RTU Protocols to MQTT
Some industrial systems need to send field data to an IoT platform or cloud-based monitoring application.
For example, a system may collect measurements from Modbus RTU devices and forward selected data to an MQTT broker. This can support remote dashboards, centralized monitoring, or data collection across multiple sites.
An industrial protocol gateway can help bridge this communication gap. Depending on its supported protocols and functions, it may connect serial Modbus devices to Ethernet networks, facilitate data exchange between industrial protocols, or forward field data to MQTT-based platforms.
When evaluating a gateway, check the exact protocols, interfaces, data mapping functions, and communication modes it supports. Not every gateway can convert every protocol or preserve all protocol-specific features.
Industrial communication manufacturers such as COME-STAR offer protocol gateways and converters for different networking requirements. The appropriate device depends on the application and the functions needed.
How to Select RTU Communication Protocols for a New or Existing System
Choosing a protocol starts with the communication requirements of the system—not simply with the most familiar protocol name.
Here are several practical points to check.
Identify the Connected Devices
List the RTUs, PLCs, meters, sensors, controllers, and supervisory systems involved.
Check which protocols and communication interfaces each device supports. If the system includes legacy equipment, confirm whether its existing protocol must be retained.
Check the Network Infrastructure
Determine whether the communication links use RS-232, RS-485, Ethernet, or a wide-area IP network.
Also consider the distance between devices, network topology, and whether the system needs serial communication, Ethernet connectivity, or remote access.
Define the Data Requirements
Consider what information needs to be exchanged:
- Periodic measurements
- Alarms and status changes
- Time-stamped events
- Remote commands
- Historical or diagnostic data
Different protocols support different communication models and data structures. Make sure the selected protocol and device implementation support the required functions.
Consider Response Time and Communication Reliability
Some applications mainly need periodic monitoring. Others require prompt reporting of alarms or changes in field conditions.
Review polling intervals, event-reporting capabilities, communication recovery behavior, and the network conditions. Do not assume that a protocol alone guarantees a particular response time or level of reliability.
Plan for Integration
If devices use different protocols, determine whether they can communicate directly or need an intermediary.
Check protocol compatibility, data mapping, supported commands, and whether important information—such as event timestamps or status indicators—can be preserved.
Review the Operating Environment
For installations in industrial environments, consider:
- Operating temperature
- EMC and electrical noise
- Power supply requirements
- Mounting and enclosure requirements
- Communication redundancy, where needed
These factors can affect the equipment used to support RTU communication, including serial converters, network devices, and protocol gateways.
Frequently Asked Questions
Is an RTU a protocol or a device?
RTU usually refers to a Remote Terminal Unit, which is a device used to collect field data and communicate with supervisory systems.
However, “Modbus RTU” refers to a specific Modbus serial communication format. The term RTU can therefore refer to different things depending on the context.
What is the difference between Modbus RTU and Modbus TCP?
Modbus RTU is commonly used over serial connections such as RS-485 or RS-232. Modbus TCP carries Modbus messages over TCP/IP networks.
Both use the Modbus application protocol, but their communication formats and network requirements differ.
Which protocols are commonly used in power system RTUs?
DNP3 and IEC 60870-5-101/104 are commonly used in power system telecontrol. IEC 61850 is also important in substation automation, particularly for communication among intelligent electronic devices and other automation equipment.
The appropriate standard depends on the system architecture and the functions required.
Can devices using different RTU protocols communicate directly?
Not necessarily. Different protocols may use different message formats, data models, and communication services.
A protocol gateway or another integration solution may be needed to translate data between them. The required conversion functions should be verified for the specific devices and application.
Can serial RTU protocols be used over Ethernet?
Yes, in some applications. A serial-to-Ethernet converter or protocol gateway may provide a way to connect serial devices to an Ethernet network.
However, simply transporting serial data over Ethernet is not the same as converting one application protocol into another. The required function depends on whether the system needs transparent serial communication or protocol-level data conversion.
Conclusion
RTU communication protocols support remote monitoring and control across power systems, industrial automation, and connected building or IoT applications.
Modbus RTU, DNP3, IEC 60870, IEC 61850, and other protocols differ in their communication models, network requirements, and data structures. Understanding these differences helps engineers assess device compatibility and plan communication between field equipment and supervisory systems.
For systems that combine different protocols, the key is to verify both network connectivity and application-level compatibility. With the right protocol support and system design, existing RTU equipment can often be incorporated into broader industrial communication networks.
Source: Adapted from COME-STAR Industrial Networking Technical Blog →
