How to Integrate Decades-Old Modbus Devices Without Regret

Modbus remains the most common language for industrial data and the simplest communication protocol to add to the drives, meters, power analyzers, weigh scales and specialty instrumentation found in every kind of process and discrete manufacturing plant. OEMs and vendors alike know that supporting Modbus provides a long-lasting communication protocol that is both inexpensive and universally supported. 

Conceived during an era prior to common factory floor device communications, Modbus arrived on the scene when PLCs used chassis-based I/O. Though Modbus has survived the transition to multi-vendor, multi-network and data-hungry PLCs, HMIs and SCADA systems, it has not evolved, yet it continues to thrive.

A primary reason Modbus systems haven’t been replaced is that upgrading and replacing these systems can be expensive with little return on investment. Essentially, the rip and replace method almost always costs more than simply integrating Modbus data from your legacy device into your updated control system or enterprise application. 

Plus, integrating Modbus into modern systems isn’t expensive. Modbus gateways from HMS, Moxa or Real Time Automation avoid the downtime, equipment costs and the significant labor of more functional and complex higher-end instrumentation. 

Though some contend that Modbus has become obsolete in today’s automation landscape, data indicates that not only isn’t Modbus obsolete, the Modbus market is expanding. A study from FutureBytes Labs, “Exploring the Dynamics of Modbus Communication Module Market: Key Insights and Trends for 2033,” indicates that the Modbus market isn’t shrinking and is expected to grow from $1.2 to $2.5 billion over the next 7 years.

Can Modbus networks be secured?

Modbus networks can be secured at the Modbus master and client devices — the devices that interface Modbus networks to the outside world. Modbus itself was never designed to be secure, as it has no authentication, encryption or access control.

Modbus TCP and RTU networks do provide a measure of security due to the Modbus architecture. A Modbus RTU slave or TCP server device only transmits on request and only transmits data values with no ability to message the client/master or any other device.  When compromised, the threat exposure varies with how well the system is segmented, what application the data influences and what other Modbus devices can be reached from the compromised device.

With that said, cybersecurity threats are more pervasive and serious in the devices that collect and manipulate Modbus data. Those PLCs, servers, HMIs, gateways and other devices have a larger threat footprint and are cybersecurity concerns, whether data is collected from Modbus TCP servers or RTU slaves.

Six rules for integrating Modbus networks in 2026

Your Modbus networks contain valuable data that is likely not critical to your control loops but important to your overall operation. These valuable data points include pressures, levels, flows, vibrations, energy and other data associated with the operation of your production systems. 

Making that data available to your PLC logic, SCADA system or enterprise applications is important for monitoring in-plant utilities, such as water, air and power. This legacy data has value when contextualized correctly.

With the importance of these data points in mind, following are six important rules for integrating Modbus network data into your operations network:

1. Identify which data (temperature, speed, energy, pressure) is available, the destination for it and the format that best suits the receiving application. Collect and transmit data you may not need today; it’s essentially free, and you’ll likely find a use for it in the future. 
2. Exclude control data from Modbus unless your PLC can act as a Modbus RTU master or TCP client. Control system data should be on faster, more highly integrated networks like EtherNet/IP or Profinet. 
3. For maximum maintainability and troubleshooting, use a single device to collect and format Modbus data. This isn’t an issue with Modbus RTU, as it can only have one master. But on Modbus TCP, which supports multiple clients, there should only be one client device connected to a Modbus server device.
4. Use a gateway or interface device to convert and type the data for easier ingestion by a receiver. Modbus devices are notorious for overlaying types on unsigned integer registers (i.e., four registers forming a floating-point value). PLCs are not where you want these translations to occur.
5. Realize that, as Modbus devices reach obsolescence, the vendors are likely to replace them with advanced security and network communications. Design your network to accommodate Ethernet capable devices in the future. 
6. Collect and store Modbus device documentation for each device on the Modbus network. Create a map of the available registers and coils along with any overlaid data formats. This information is vital for future troubleshooting.

3 Modbus network integration architectures

In 2026, there are three typical architectures for integrating Modbus networks: Protocol gateways that provide integration with PLCs; historians that archive Modbus data for maintenance, troubleshooting and compliance; and edge gateways that provide integration with enterprise and cloud applications.

What to avoid when integrating Modbus networks

Some of the potential problems, in priority order, are:

1. Missing documentation. Legacy devices, designed and built before the Internet can be very difficult to find. Without the right documentation, it’s impossible to know what data is available and how it is formatted. 
2. Register alignment, scaling and endianness issues. This goes with lack of proper documentation. Document the endianness, format and units for every register in your process. Know the destination of that data and if the value must be converted and retyped for the destination application. This is especially important when moving Modbus data into a PLC. 
3. Termination and wiring with Modbus RTU. Failed terminations, missing terminating resistors and the wrong wiring has doomed many a Modbus project.  Inspecting, tightening and verifying terminating resistors pays huge dividends.
4. Polling rates vs. device response limits. You may configure your Modbus master or client to poll every 20 msec, but if you have several devices that take 100 msec to respond, your data ingestion will be different from what you’ve planned. 
5. Embedding Modbus logic deep in PLC programs. PLCs are not protocol gateways, making this action an insidious mistake. Control logic updates can change how your Modbus network is polled. Modbus logic updates can alter your control logic. Maintainability is often severely compromised when using Modbus out of a PLC. 
6. No diagnostics. Not having a diagnostic process in place severely inhibits troubleshooting. Make sure your Modbus master or client (protocol gateway, historian or edge gateway) provides diagnostic data when data goes missing. 
7. Troubleshooting tools. There are lots of Modbus analyzers, protocol diagnostic tools and simulators available. Have one or more of them ready to go when a problem strikes. 
8. Vendor-specific function code behavior. Some Modbus vendors have adopted non-standard function codes, register formats, bit configurations and other features. Verify that your Modbus master or client can use those function codes or data formats. 
9. Treating Modbus as being temporary for years. Modbus devices work and have valuable data. That does not mean that they should be in your system 10 years from now. Over time, move to meters, scales and other devices that offer greater connectivity and easier integration.

• The Biggest Trends in Industrial Connectivity 
• Single Pair Ethernet: A Two-Wire Cable is Transforming Industrial Communications 
• Insights on Managing I/O Device Diversity and Data Overload in Industrial IoT