What Is Modbus RTU?
Modbus RTU is a serial communication protocol widely used in industrial automation for data exchange between controllers, sensors, and actuators. It operates over RS-232 or RS-485 at baud rates from 9600 to 115200 bps, with a single master and up to 247 slaves per bus. The protocol uses a 16-bit CRC for error checking and supports function codes for reading coils (0x01), holding registers (0x03), and writing registers (0x06/0x10). In practice, RS-485 multidrop networks commonly run Modbus RTU over 120-ohm terminated twisted pair, reaching 1000+ meters at 9600 baud.
Key Takeaways
- Real-world engineering considerations from 10+ years of STM32 development
- Practical tradeoffs between performance, cost, and time-to-market
- Direct experience across industrial, energy, and IoT deployments
Frequently Asked Questions
Modbus RTU vs Modbus TCP: which should I use?
RTU is serial (RS-485, lower cost per node, longer distance) use for field devices. TCP runs over Ethernet, use when existing IP infrastructure exists. Many gateways bridge both protocols.
What is the maximum distance for Modbus RTU?
With RS-485 at 9600 baud and proper termination, reliable communication up to 1200 meters is achievable. At higher baud rates (115200+), keep cable runs under 100 meters.
Can I have multiple masters?
Standard Modbus RTU supports only a single master. Multi-master requires higher-layer arbitration or a gateway with failover detection.
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