Modbus is an open communication protocol developed by Modicon (now part of Schneider Electric). Featuring open specifications, free authorization and simple implementation, it has become the most widely used serial communication protocol in factory automation. Typical slave devices include temperature and humidity meters, I/O modules, power meters, inverters, motor units and other industrial field devices.
Modbus RTU Working Principle: Master-Slave Architecture and Polling Mechanism
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1. 1 Master Device: Devices such as PLCs, data acquisition gateways, host computers and HMI touch panels, which are the only devices authorized to initiate communication on the bus.
2. Multiple Slave Devices: Temperature and humidity transmitters, sensors, meters, inverters, namely Slave 1 to Slave n shown in the figure.
3. The physical link typically adopts RS485 half-duplex differential bus.
4. Core rule: Slave devices cannot actively transmit data! All communication requests are initiated by the master device.
Communication Mode: Request-Response Polling
1. The master device broadcasts and sends a Request Frame on the bus;
2. All slave devices on the bus receive the message;
3. Each slave device identifies the slave address in the frame. Only the device with a matching address processes the instruction, while all other slaves ignore the message directly;
4. The selected slave device organizes valid data and returns a Response Frame to the master;
5. Only one pair of request-response communication can be executed on the bus at a time. After one communication cycle is completed, the master initiates a query to the next slave device.
Tech Tips: Slave devices are incapable of active data upload. For industrial temperature and humidity collection scenarios, the master polls each sensor cyclically and sequentially to acquire data.
Advantages of Modbus RTU
1. Multi-drop master-slave bus: Dozens of temperature and humidity sensors can be connected in series via a single RS485 cable, reducing wiring costs and perfectly adapting to multi-point distributed temperature measurement scenarios.
2. High transmission efficiency of binary RTU mode: Compared with the Modbus ASCII protocol, it lowers bus load and enables faster refresh rates for multi-point polling.
3. Standardized register model: Parameters such as temperature, humidity and alarm values are uniformly mapped to registers, realizing interoperability of transmitters from different manufacturers.
4. The RS485 differential bus features strong anti-interference capability and long-distance transmission, suitable for factory workshops, cold storage, greenhouses and other complex industrial environments.
5. CRC16 checksum ensures data transmission reliability and prevents temperature and humidity measurement errors caused by on-site interference.
Related Products
More Resources
* RENESAS Modbus RTU Application Note
Extended Technical Topics
- Why does a simple industrial control project require Modbus, CAN, and EtherCAT?
- Why CAN alone is insufficient, and a complete CANopen architecture is mandatory for motion control?
- Fundamentals of CANopen Motion Control Safety Procedures
- How Does EtherCAT Achieve Low Latency?
- EtherCAT Low-Latency Communication: Physical Layer Reference Circuit
- Modbus RTU Working Principle: Master-Slave Architecture and Polling Mechanism
- Modbus RTU Communication Process