What is
MODBUS?
Understanding the Modbus Protocol in Industrial Automation
Introduction to Modbus
If you’re wondering what is Modbus, you’re not alone. Modbus is a widely used industrial automation communication protocol designed to enable seamless data exchange between programmable logic controllers (PLCs), sensors, actuators and monitoring systems. Originally created in 1979 by Modicon (now Schneider Electric), the Modbus protocol has since evolved into one of the most trusted standards in the world of industrial automation.
As an open protocol, Modbus explained in simple terms is a method for transmitting information over serial lines (Modbus RTU) and Ethernet networks (Modbus TCP/IP). It is widely used because it is flexible, vendor-neutral and well-documented—making it an ideal communication solution for modern and legacy systems alike.
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Modbus Blogs
Organizations/Memberships
RTA is a proud member of the Modbus Organization. For more information, visit their site: modbus.org
Why Modbus is Important in Industrial Settings
Modbus plays a critical role in streamlining industrial operations. From PLC control to device monitoring, Modbus PLC communication is the backbone of many industrial processes. Because it’s an open standard, manufacturers around the world continue to implement the Modbus protocol into new equipment and systems.
Here’s why it continues to thrive:
- It reduces wiring and simplifies installation in PLC networks.
- It integrates well with other industrial protocols through gateways.
- It supports both serial (Modbus RTU) and Ethernet (Modbus TCP) communications.
- It is ideal for industrial automation communication protocol needs across diverse environments.
Types of Modbus Protocols
Understanding the differences between the types of Modbus protocols is essential when choosing the right solution.
- Modbus RTU: This is a serial protocol typically transmitted over RS-232 or RS-485. Modbus RTU is compact, binary-based and highly efficient for data transfer in environments where timing is critical.
- Modbus ASCII: Like RTU, but the data is encoded in ASCII characters. It’s human-readable but slower and less common in modern applications.
- Modbus TCP/IP: Known simply as Modbus TCP, this version operates over Ethernet. It’s widely used for networked devices and allows for faster communication and support for multiple clients.
Each variant of the Modbus protocol addresses unique connectivity needs in industrial automation. Whether you need to wire a few devices together in a plant or create an enterprise-wide PLC communication strategy, there’s a Modbus format that fits.
Modbus Function Codes
Modbus function codes are at the heart of Modbus PLC communication. These codes tell Modbus-enabled devices what actions to perform. Here are common ones:
- FC01: Read Coils
- FC02: Read Discrete Inputs
- FC03: Read Holding Registers
- FC05: Write Single Coil
- FC06: Write Single Register
- FC15: Write Multiple Coils
- FC16: Write Multiple Registers
By using these function codes, a Modbus client (or master) can collect sensor data, control actuators, and adjust system parameters across any Modbus RTU or Modbus TCP network.
Modbus Device Types
In the Modbus protocol, two main device types are defined:
- Clients (Masters): These devices initiate communication. In a Modbus PLC communication setup, this is often a PLC or SCADA system.
- Servers (Slaves): These devices respond to requests and return data. Examples include drives, sensors and meters.
The clarity of this master/slave architecture makes Modbus one of the easiest industrial automation communication protocols to understand and deploy.
Modbus Message Structure
Each Modbus message consists of a function code, data fields and error-checking components. This structure remains consistent whether you’re using Modbus RTU, Modbus ASCII or Modbus TCP/IP.
Fields include:
- Function Code
- Start Address
- Quantity of Registers/Coils
- Byte Count
- CRC (RTU) or LRC (ASCII) for error detection
- MBAP Header (TCP)
Modbus Use Cases
Wondering how this protocol is used? Here are some real-world Modbus use cases:
- Connecting legacy PLCs and HMI systems with Modbus RTU
- Integrating smart energy meters using Modbus TCP/IP
- Monitoring remote tanks with cellular gateways using Modbus
- Retrofitting legacy automation equipment for IIoT using Modbus explained integration techniques
The protocol’s simplicity makes it the go-to solution for many industrial communication scenarios.
Connecting PLCs with Modbus
PLCs use Modbus to communicate with devices like VFDs, sensors, I/O blocks and other PLCs. In a typical Modbus PLC communication setup:
- A master PLC sends a query
- The slave device responds with data or status
- This cycle repeats continuously for real-time updates
Whether through Modbus RTU or Modbus TCP, this structure forms the backbone of many modern automation systems.
How RTA Gateways Help with Modbus Integration
Real Time Automation offers industrial-grade gateways that make it easy to convert Modbus RTU to Modbus TCP, or bridge Modbus to protocols like BACnet, EtherNet/IP and Profinet. These plug-and-play devices eliminate programming complexity and ensure fast, reliable integration.
If you’re working with Modbus protocol integration projects—especially ones involving different types of industrial automation communication protocols—RTA’s solutions provide the fastest path to connectivity.
FAQs
Q: What is Modbus used for?
A: Modbus is used to connect industrial automation equipment and ensure devices can exchange data. It’s the preferred industrial automation communication protocol in factories, water plants, and energy systems.
Q: What’s the difference between Modbus RTU and Modbus TCP?
A: Modbus RTU uses serial connections and is common for point-to-point wiring. Modbus TCP uses Ethernet and supports larger networks and faster speeds.
Q: What is Modbus explained in simple terms?
A: It’s a communication language that allows industrial devices to talk to each other. Devices request and send information using function codes, either through serial (Modbus RTU) or Ethernet (Modbus TCP).
Q: How does Modbus PLC communication work?
A: A master PLC sends a command using the Modbus protocol. The slave device responds with the requested data. This process enables automation systems to control, read and respond in real-time.
Q: Is Modbus still relevant in today’s automation world?
A: Absolutely. Modbus RTU and Modbus TCP remain essential in industrial automation, especially when working with legacy devices and mixed-protocol environments.
Q: Why is Modbus considered a top industrial automation communication protocol?
A: Its simplicity, open standard, and compatibility make it a leading solution for PLC communication. Whether using Modbus RTU or TCP/IP, the protocol enables streamlined integration across systems.
Why Modbus Still Matters
Modbus isn’t going away anytime soon. It’s one of the most widely used, easiest-to-implement protocols available for industrial automation. With strong support for Modbus RTU, Modbus TCP and flexible gateway solutions, it provides a seamless pathway for connecting and controlling modern automation equipment.
