How to convert Modbus RTU to OPC UA
How to Convert Modbus RTU to OPC UA
In the evolving landscape of industrial automation, seamless communication between legacy field devices and modern SCADA systems is essential. Modbus RTU, a long-standing serial communication protocol, remains prevalent in many industrial facilities. Meanwhile, OPC UA (Open Platform Communications Unified Architecture) has become the de facto standard for modern, secure, and interoperable data exchange. Converting Modbus RTU to OPC UA enables data integration across traditional and next-generation systems.
This guide explores the technical, architectural, and implementation aspects of converting Modbus RTU to OPC UA, enabling manufacturers and system integrators to modernize operations while leveraging existing investments.
nderstanding the Protocols
Modbus RTU Overview
Modbus RTU is a serial protocol developed by Modicon in 1979. It operates over RS-232 or RS-485 and is widely used due to its simplicity and reliability. Modbus RTU communicates in a master-slave format, where the master polls slaves for data.
Transmission Mode: Binary (RTU)
Communication: Half/Full Duplex over serial lines
Data Format: Register/coil-based
Limitations: No encryption, no discovery, limited addressing
OPC UA Overview
OPC UA is a platform-independent, service-oriented protocol for industrial automation and data exchange.
Transport: TCP/IP, HTTPS, WebSockets
Features: Discovery, security, data modeling, subscriptions
Communication Model: Client-server and publish-subscribe
Extensibility: Supports complex data structures and semantics
Use Cases for Modbus RTU to OPC UA Conversion
Integrating legacy Modbus RTU sensors and controllers into modern OPC UA-based SCADA or MES systems
Cloud integration using OPC UA clients in edge devices
Enhancing security and manageability in brownfield installations
Data standardization across heterogeneous devices
Conversion Architecture
Physical Connectivity
To convert Modbus RTU (serial) data into OPC UA (Ethernet-based), an intermediary device or software gateway is required.
Hardware Requirements:
Serial port (RS-232/RS-485)
Ethernet port (for OPC UA communication)
Gateway Functionality
The gateway must:
Act as a Modbus master (polling RTU slaves)
Translate Modbus registers to OPC UA nodes
Serve data to OPC UA clients securely
Network Architecture
Modbus RTU Devices <–RS-485–> OPC UA Gateway <–Ethernet–> SCADA/MES
Selecting the Right Gateway
Hardware Gateways
Softing edgeGate
ProSoft PLX51-UA-MB
HMS Anybus Communicator IIoT
ICP DAS UA-5231
Software-Based Gateways
Unified Automation SDKs
Kepware KEPServerEX with Modbus and OPC UA plug-ins
Matrikon OPC UA Tunneler
Open-Source Options
Open62541 (OPC UA library)
libmodbus (Modbus stack)
Node-RED with Modbus & OPC UA nodes
Implementation Steps
Prepare Modbus RTU Device Map
Understand the Modbus slave address, function codes, and register mappings.
Configure the Gateway
Set up serial parameters (baud rate, parity, stop bits)
Add Modbus polling configuration (addresses, intervals)
Define OPC UA namespace and node hierarchy
Test Modbus Communication
Use tools like ModScan or QModMaster
Ensure reliable polling and data integrity
Expose OPC UA Nodes
Map Modbus registers to OPC UA variables
Assign meaningful tag names, data types, and metadata
Connect to OPC UA Client
Use UaExpert or Ignition Designer to test connectivity
Browse the namespace, read/write values
Implement Security (Optional)
Enable user authentication, encryption (Basic256Sha256)
Restrict node access by role
Best Practices
Use structured namespaces (e.g., Devices/Boiler1/Temperature)
Enable diagnostic logs and monitor communication errors
Plan for retry logic and device timeouts
Use secure networks (VPN, VLANs) for OPC UA traffic
Advanced Features and Extensions
MQTT and Cloud Integration
Some gateways can publish OPC UA data to MQTT brokers for cloud applications (e.g., AWS IoT, Azure IoT Hub).
Companion Specification Support
For advanced applications, support for industry-specific OPC UA Companion Specs (e.g., PA-DIM, AutoID) can be added.
Edge Analytics
Gateways may support scripting, threshold alarms, or local storage for edge computing scenarios.
Troubleshooting Tips
Check serial wiring and termination
Monitor baud rate mismatches
Ensure the Modbus slave responds to polling
Use OPC UA logs to identify certificate or namespace issues
Conclusion
Converting Modbus RTU to OPC UA bridges the gap between legacy field devices and next-generation automation systems. Whether through hardware gateways or software stacks, the conversion enables secure, standardized, and scalable data communication. With careful planning and configuration, organizations can preserve existing investments while unlocking the potential of OPC UA for smart manufacturing, IIoT, and Industry 4.0.
This guide provides a blueprint for engineers, integrators, and decision-makers embarking on the journey of Modbus to OPC UA migration. As more SCADA systems adopt OPC UA natively, mastering this conversion becomes an essential skill in the modern automation toolkit.
Appendix A: Sample Node Mapping Table
Modbus Address
Function Code
OPC UA Node Name
Data Type
40001
03 (Read Holding)
Boiler1.Temp
Float32
40002
03
Boiler1.Pressure
Float32
00001
01 (Read Coil)
Pump1.Status
Boolean
Appendix B: Recommended Tools
UaExpert – OPC UA client for testing
QModMaster – Modbus RTU testing tool
Wireshark – Packet capture and analysis
Open62541 – OPC UA open-source stack
libmodbus – Modbus RTU/TCP library
ANC-300e: Ethernet/IP & Modbus TCP to Serial Modbus RTU, DF1 Converter
The ANC-300e has the combined performance as an Ethernet and RS-485 serial converter. The gateway behaves like a protocol converter/translator that allows difficult data interchanges and connections between incompatible networks. Protocols include:
RS485 serial Protocols:
Modbus RTU, Modbus Sniffer, A.O. Smith AIN, A.O. Smith PDNP, BACnet MS/TP Client, BACnet MS/TP Server, TCS Basys Master, MSA Chillgard Monitor, DMX 512-Master, DMX-512 Slave, M-Bus Master, Metasys N2 Master, Metasys N2 Slave, Siemens FLN Master, Siemens FLN Slave, Sullair Master, Toshiba ASD Master, Toshiba PLC Master, DF1.
Ethernet Protocols:
Ethernet/IP Client/Server, AB CSP Ethernet Client/Server, Modbus TCP/IP, Profinet IO, BACnet/IP BBMD, BACnet/IP Client, BACnet/IP Server, Baumer VeriSens Client, CC-Link IE SLMP Client, MELSEC Client/Server, Generic Socket Client/Server & GreenFumeHood Client, Modbus TCP.
ANC-300e is an easily configurable Ethernet / RS-485 converter gateway, that provides integrated communication between all the above common industrial and commercial communication networks. As part of the ANC Gateway Converter Series, the ANC-300e provides a common platform which is convenient and easy to setup in all products.
The Automation Network ANC-300e Ethernet & Fieldbus Gateway Converter allows data to be seamlessly transferred between Ethernet to Ethernet and Ethernet to RS-485 networks with simple configuration.
Coming Soon: Need to get data from your EtherNet/IP or Modbus TCP/IP-based devices to your OPC UA clients? This ANC-300e additional functionality is coming shortly.