How We Can Convert Modbus RTU to Modbus TCP

In industrial automation, Modbus is one of the most common communication protocols used to connect devices such as sensors, controllers, meters, and SCADA systems.
Over the years, two major versions of Modbus have been developed — Modbus RTU, which communicates over serial lines, and Modbus TCP, which communicates over Ethernet.

As technology evolves, many industries face the challenge of integrating older Modbus RTU devices with modern Modbus TCP systems. Fortunately, this conversion is possible — and relatively simple — because both protocols share the same foundation.

This article explains why we can convert Modbus RTU to Modbus TCP and how the conversion process works in real-world industrial networks.

Understanding Modbus RTU and Modbus TCP

Before explaining the conversion, it’s important to understand the difference between these two communication types.

Feature	Modbus RTU	Modbus TCP
Communication Medium	Serial (RS-232 / RS-485)	Ethernet (TCP/IP)
Data Format	Binary frames	Encapsulated Modbus messages in TCP packets
Addressing	Slave ID (1–247)	IP address + Unit ID
Speed	Up to 115.2 kbps	Up to 1 Gbps or more
Topology	Bus or point-to-point	Star or network-based

Both RTU and TCP use the same Modbus protocol structure — meaning they share function codes, register types (coils, inputs, holding registers), and data mapping principles.

This shared structure is the key reason why converting between the two is technically possible.

Why We Can Convert Modbus RTU to Modbus TCP

The conversion between Modbus RTU and Modbus TCP is possible because the Modbus protocol itself remains unchanged — only the communication layer is different.

Think of it like changing the language’s “envelope,” not the message inside it.
The Modbus RTU frame is wrapped in a serial communication format, while Modbus TCP simply wraps the same data inside a TCP/IP packet.

So, a Modbus gateway can easily:

1. Read Modbus RTU messages from serial devices.
2. Encapsulate those messages into Modbus TCP packets.
3. Send them over Ethernet to a Modbus TCP client (like a SCADA system).

Because the data model is identical, the gateway doesn’t need to translate the actual Modbus commands — it only changes the way they are transmitted.

Why Convert Modbus RTU to Modbus TCP

There are several practical reasons why industries perform this conversion:

1. Integration of Legacy Devices

Many older machines and instruments still use Modbus RTU. Converting them to Modbus TCP allows them to work with modern control systems, PLCs, and SCADA platforms.

2. Longer Communication Distance

RS-485 serial communication is limited to about 1.2 km. Ethernet can transmit data across buildings, plants, or even remotely via VPN or the Internet.

3. Faster Communication

Modbus TCP runs over high-speed Ethernet networks, making data exchange faster and more reliable — especially when multiple devices are polled simultaneously.

4. Remote Monitoring and Control

With Modbus TCP, you can monitor and control field devices from anywhere on the network, enabling remote diagnostics and industrial IoT (IIoT) applications.

5. Easier System Maintenance

Ethernet-based systems are easier to configure, expand, and troubleshoot compared to traditional serial networks.

How the Conversion Works

A Modbus RTU to Modbus TCP gateway acts as a bridge between serial and Ethernet networks.

Here’s how the communication flow works:

1. A Modbus TCP client (for example, a SCADA system) sends a request over Ethernet to the gateway.
2. The gateway extracts the Modbus data from the TCP frame.
3. It then sends that same request to the Modbus RTU device through the RS-485 or RS-232 serial port.
4. The RTU device processes the request and sends back a response.
5. The gateway wraps that response into a Modbus TCP packet and sends it back to the client.

This process happens in milliseconds, providing seamless real-time communication between devices that were never originally designed to work together.

Conclusion

We can convert Modbus RTU to Modbus TCP because both protocols share the same core structure — the only difference lies in how they transmit data. Using a Modbus gateway, the conversion is simple, fast, and efficient.

This conversion enables legacy Modbus RTU devices to communicate seamlessly with modern Ethernet-based control systems, providing better performance, scalability, and remote access capabilities.

In today’s connected industrial world, bridging the gap between serial and Ethernet communication is no longer a luxury — it’s a necessity for building smart, future-ready automation systems.

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