What is Modbus?
Modbus is an open communication protocol developed in 1979 by Modicon (now part of Schneider Electric). It is widely used in industrial environments for communication between electronic devices, such as PLCs, Sensors, Actuators and HMIs.
Modbus is simple, robust and broadly supported, which makes it one of the most widely used industrial protocols in the world.
🔧 What does Modbus do?
Modbus enables devices to exchange information using a master/slave or client/server structure.
🧱 Modbus variants
| Variant | Description |
|---|---|
| Modbus RTU | Serial communication via RS-232 or RS-485. Binary transmission (compact and fast). |
| Modbus ASCII | Serial communication using ASCII characters. Easy to debug, but slower. |
| Modbus TCP/IP | Modbus over Ethernet (TCP). Suitable for modern networks. |
| Modbus Plus | Faster, Schneider’s proprietary variant. Less open. |
🔄 How does Modbus work?
- The master sends a request with a function code (e.g. “read registers”).
- The slave returns a response with the requested data.
- Data is usually exchanged in the form of registers (16-bit), coils or discrete inputs.
🏭 Examples of use
- Sensor readout (e.g. temperature or pressure)
- PLC-to-PLC communication
- Driving valves and motors
- Energy management and building management systems
📦 Typical Modbus function codes
| Code | Function |
|---|---|
| 01 | Read discrete outputs (coils) |
| 02 | Read discrete inputs |
| 03 | Read holding registers |
| 04 | Read input registers |
| 05 | Write single coil |
| 06 | Write single holding register |
⚠️ Limitations of Modbus
- No built-in security (no encryption or authentication)
- Limited speed and bandwidth (especially for serial versions)
- No standard for complex data types (e.g. floating point or strings)
- Generally only one master per network (in RTU/ASCII)
🔐 Modbus and cybersecurity
Because Modbus has no encryption or access control, it is vulnerable to attacks, especially over TCP/IP:
- Use a VPN or industrial Firewall
- Segment networks (e.g. with the Purdue Model)
- Restrict physical and network access
📌 In summary
Modbus is a simple, broadly supported protocol for communication between industrial devices. Despite its limitations, it remains widespread in automation.