As industrial automation systems continue to evolve, many facilities face the same challenge: how do you connect legacy Modbus RTU/ASCII devices to modern Ethernet-based Modbus TCP networks?

The good news is that you don’t always need to replace your existing equipment. In most cases, there are two proven approaches:

• Transparent Transmission (Serial over Ethernet)
• Protocol Conversion (Modbus Gateway)

Although both connect serial Modbus devices to Ethernet, they work quite differently and are designed for different applications.

Option 1: Transparent Transmission

Transparent transmission, often called Modbus RTU over TCP, simply forwards serial data through an Ethernet network without interpreting or modifying the protocol.

Think of it as creating a virtual serial cable over IP. The Modbus serial-to-Ethernet converter packages the original RS-232, RS-422, or RS-485 data into TCP packets, while the host software remains responsible for processing every Modbus message.

Advantages

• Easy to deploy
• Lower hardware cost
• Keeps the original communication logic unchanged

Limitations

• The host system must perform all protocol parsing.
• Communication still follows Modbus RTU’s sequential polling mechanism, which becomes slower as more slave devices are added.
• Additional software may be required if the host only supports Modbus TCP.

For smaller systems or projects where existing software already understands Modbus RTU, transparent transmission is often the simplest solution.

Option 2: Protocol Conversion

A Modbus gateway takes a different approach. Instead of simply forwarding data, it converts Modbus RTU/ASCII frames into Modbus TCP (and vice versa), allowing Ethernet-based PLCs, HMIs, or SCADA systems to communicate directly with serial devices.

Many industrial gateways also include intelligent features such as:

• Automatic polling of slave devices
• Local data caching
• Pre-reading frequently accessed registers
• Faster response to Modbus TCP requests

Because much of the communication work happens inside the gateway, the host system becomes much simpler and communication efficiency improves significantly, especially in larger networks.

The trade-off is slightly higher hardware cost and more configuration during deployment.

Which Solution Should You Choose?

There isn’t a universal answer—it depends on your application.

Choose transparent transmission if you:

• Only need to extend several serial devices over Ethernet
• Already have software that supports Modbus RTU
• Want the lowest-cost implementation

Choose protocol conversion if you:

• Connect multiple Modbus slave devices
• Use SCADA, PLCs, or HMIs that only support Modbus TCP
• Want faster response times and simpler host software
• Plan to expand your system in the future

Other Selection Factors

Regardless of which approach you choose, several practical considerations can affect long-term reliability:

• Support for the required serial interface (RS-232, RS-422, or RS-485)
• Sufficient number of serial ports
• Compatible baud rates and serial communication settings
• Industrial-grade design for harsh environments, including wide operating temperature, EMC protection, and rugged enclosures

These factors often have just as much impact on system stability as the communication method itself.

Final Thoughts

Both transparent transmission and protocol conversion remain widely used in industrial automation. Transparent transmission offers a straightforward, economical way to extend serial communication over Ethernet, while protocol conversion provides greater performance and easier integration for modern Modbus TCP systems.

Many industrial networking manufacturers now offer complete product portfolios covering both serial-to-Ethernet converters and Modbus gateways, allowing engineers to choose the solution that best matches their application instead of forcing a one-size-fits-all approach.

Source: Adapted from COME-STAR Industrial Networking Technical Blog → https://www.come-star.com/blog/modbus-tcp-to-modbus-rtu-ascii/