· Eduardo Vieira · Industrial Automation · 2 min read
Modbus RTU vs Modbus TCP: Serial vs Ethernet Explained
Dive into the technical differences between Modbus RTU and Modbus TCP and learn when to choose each mode for your industrial network.
Modbus RTU vs Modbus TCP: Serial vs Ethernet Explained
Overview
Modbus RTU and Modbus TCP are two common variants of the Modbus protocol. RTU operates over serial lines, while TCP leverages Ethernet networks. Each mode has its own advantages, challenges, and ideal use cases.
Modbus RTU (Serial RS-485/RS-232)
- Communication over RS-485 or RS-232 physical layer.
- Binary framing with CRC-16 error checking.
- Master polls one slave at a time in a half-duplex network.
- Lower bandwidth (up to 115.2 kbps) and higher latency compared to Ethernet.
- Simple wiring: point-to-point or multi-drop (up to 32 devices on RS-485).
Use Cases:
- Legacy installations and equipment with serial ports.
- Short distances (≤1200 meters for RS-485).
- Environments where Ethernet infrastructure is not available.
Modbus TCP (Ethernet)
- Encapsulates Modbus frames in TCP/IP packets (port 502).
- Full-duplex communication enabling parallel queries.
- Higher bandwidth (10/100/1000 Mbps) and lower latency.
- Supports larger networks and longer distances via standard Ethernet switches.
- Unit Identifier field allows bridging to serial backplanes.
Use Cases:
- Modern networks with existing Ethernet infrastructure.
- Large-scale installations requiring fast polling rates.
- Integration with IT systems and remote access over IP.
Key Differences
| Feature | Modbus RTU | Modbus TCP |
|---|---|---|
| Physical Layer | RS-485 / RS-232 serial | Ethernet (TCP/IP) |
| Framing | Binary + CRC-16 | TCP packet + MBAP header |
| Bandwidth | ≤115.2 kbps | 10–1000 Mbps |
| Latency | Higher, dependent on polling | Lower, parallel operations |
| Topology | Multi-drop, daisy-chain | Star, ring, mesh |
| Error Checking | CRC-16 | TCP checksum + CRC inside |
Choosing the Right Mode
- Select RTU for simplicity and where serial is sufficient.
- Choose TCP for speed, scalability, and integration with Ethernet.
- Gateways can bridge RTU devices to Ethernet networks, offering hybrid solutions.
Conclusion
Understanding the trade-offs between Modbus RTU and TCP is essential for designing efficient and reliable industrial networks. Use RTU for legacy or low-bandwidth scenarios, and leverage TCP for modern, high-performance applications.