Modbus – A Fundamental Communication Protocol in Industrial Automation
Modbus – Universal Communication Protocol in Industry
In the age of rapid industrial growth and automation, reliable and effective communication between devices is becoming increasingly important. In a world where PLC controllers, sensors, meters, and relays must cooperate in real time, a common language is needed—one that is simple, open, and flexible enough to meet growing demands. One such language, which has maintained its leading position in industrial automation for over four decades, is Modbus - a protocol that, despite its simplicity, has stood the test of time and continues to be widely used in modern installations.
Modbus has become a standard for communication between industrial devices. Its key strengths include its clear frame structure, ease of implementation, and openness, which allows it to be used without purchasing a license. As a result, Modbus is found not only in large industrial facilities but also in HVAC (Heating, Ventilation, and Air Conditioning) systems, BMS (Building Management System) applications, power plants, and even hobby and educational projects.
History and Evolution
The Modbus protocol was created in 1979 as proprietary technology by Modicon (later Schneider Electric) for use in PLC controllers. Over time, it became a standard in industrial automation due to its simplicity and openness. In the 1990s, Modbus was released as a free standard, attracting many industrial equipment manufacturers. In 2004, Schneider Electric transferred ownership of Modbus to the Modbus Organization (Modbus-IDA), making it a truly open protocol available to the user community. Over the decades, Modbus has evolved from the original ASCII and RTU formats, through many proprietary variants, to the current Modbus TCP/IP. Today, the standard also defines new profiles (e.g., Modbus/UDP) and diagnostic extensions. Despite the emergence of newer protocols, Modbus remains a cornerstone of automation: it is the “oldest” industrial automation protocol still in use across thousands of devices from various manufacturers. The transition from serial bus to Ethernet allowed Modbus to be integrated into SCADA and IoT systems without requiring changes to existing field devices.
Technical Fundamentals
Modbus operates in a client-server (master-slave) model, where the client initiates requests and the server responds. The protocol defines four basic data types:
- Coils (Output Bits) – 1-bit information acting as a read/write switch
- Discrete Inputs (Input Bits) – 1-bit read-only data
- Input Registers – 16-bit read-only data
- Holding Registers – 16-bit read/write data
Communication occurs through frames containing the device address, function code, data, and a checksum (CRC or LRC).
Variants
Modbus RTU
Uses RS-232 or RS-485 serial interfaces; data is transmitted in binary (8-bit) format with a CRC checksum. This ensures high transmission efficiency. RTU frames must be sent as continuous sequences without long pauses.
Modbus ASCII
Similar to RTU but with data encoded in ASCII format and an LRC checksum. Each data byte is transmitted as two ASCII characters, allowing for longer pauses (up to 1 second) between characters without data loss.
Modbus TCP/IP
An implementation of Modbus for Ethernet networks, using the TCP/IP protocol. A standard RTU frame (without a checksum) is encapsulated in a TCP packet (port 502). Devices can operate as peers within an IP network. This allows Modbus to function in LAN/WAN environments, increasing transmission range and speed.
These variants make Modbus suitable for both simple local systems and large industrial networks.
Applications
Modbus is highly versatile and used in many automation fields. In SCADA systems (Supervisory Control and Data Acquisition), it collects data from distributed sensors and controllers, enabling process monitoring in power plants, transmission networks, and industrial facilities. For example, in the oil and gas industry, Modbus devices monitor pipeline pressure and flow, improving safety and operational efficiency. In HVAC, the protocol is used in building controllers and actuators, enabling the BMS system to read temperature measurements, control fans, or adjust heating valves. Modbus is also widely used in food automation and packaging machinery, where devices exposed to harsh environments send readings (e.g., oven temperatures, conveyor speeds) to higher-level control systems.
In practice, Modbus typically handles tasks like monitoring signals (e.g., temperature, pressure, level sensors) and controlling actuators (e.g., pumps, motors, valves). As an open standard, it allows for easy integration of equipment from different brands, which is essential in production line upgrades and expansions.
Comparison with CAN, Profibus, and BACnet
These protocols differ in terms of performance, complexity, and typical use cases.
CAN (Controller Area Network)
Is a reliable, bidirectional serial bus protocol, widely used in automotive and vehicle automation. It offers deterministic message delivery and strong noise immunity. However, its maximum speed of ~1 Mb/s is only achievable over short distances (tens of meters). At longer distances, speed decreases, limiting CAN to local bus applications. Compared to CAN, Modbus RTU is slower (typically up to 115 kb/s over RS-485), but simpler to implement and capable of addressing more devices on a single line (up to 247 addresses).
Profibus DP
Developed in the 1990s, is a fast and feature-rich fieldbus protocol. It supports transmission speeds up to ~12 Mb/s (RS-485), deterministic access methods (master–slave or token-passing), and very short data refresh times. However, its specification is highly complex, spanning thousands of pages describing various modes and services. Profibus is therefore considered a "strong node" in machine automation (Industry 4.0), offering high throughput and redundancy, at the cost of more complex configuration. Modbus, by contrast, is "simpler"—with a specification under 100 pages and easier deployment on standard PLCs. In practice, the choice depends on the application: Profibus fits fast production networks, Modbus is better suited for distributed or less demanding systems.
BACnet
Is an open standard (ASHRAE) for building automation (HVAC, safety, lighting). It supports both RS‑485 lines (BACnet MS/TP) and Ethernet/IP networks (BACnet/IP). Unlike Modbus or CAN, BACnet includes a rich set of network services and objects (e.g., mailboxes, advanced alarms). It's widely used in building management systems, though it carries more functional overhead and isn’t optimized for very short cycle times. In a building automation industry survey, 62% of installations used BACnet, 40% LonWorks, and 30% Modbus—showing that Modbus also plays a role in BMS, mainly due to its simplicity and compatibility with industrial equipment.
Modbus Devices Available in Our Store
Redisage offers a wide range of devices compatible with the Modbus protocol, enabling integration and automation of industrial processes.
Converters and Communication Gateways
These devices convert signals between different protocols and interfaces, allowing older systems to integrate with modern networks and enabling simple network management.
Controllers and I/O Modules
Digital and analog I/O modules with Modbus support allow easy expansion of automation system capabilities.
With these devices, integrating and expanding automation systems becomes easier and more efficient.
Summary
Modbus is a proven and reliable communication protocol that, thanks to its simplicity and flexibility, has found wide application in industrial automation. The availability of various Modbus-compatible devices from Redisage.com enables the creation of comprehensive, scalable automation systems tailored to individual user needs.
If you're planning to modernize or expand your system, it's worth considering the use of the Modbus protocol and the broad range of devices on the market that support this standard.