Modbus RTU vs Modbus TCP — Key Differences Explained
Key Takeaway
Modbus RTU uses serial RS-485 with binary encoding and CRC. Modbus TCP uses Ethernet on port 502 with the same data model. Both use identical register addressing and function codes.
Quick Answer
Modbus RTU and Modbus TCP share the same data model and function codes but differ in physical and transport layers. RTU uses serial RS-485 with CRC. TCP uses Ethernet on port 502 with TCP/IP error handling.
Comparison
| Feature | Modbus RTU | Modbus TCP |
|---|---|---|
| Physical Layer | RS-485 or RS-232 | Ethernet |
| Encoding | Binary | Binary + MBAP header |
| Error Check | CRC-16 | TCP (no Modbus CRC) |
| Addressing | Slave address 1-247 | IP address + Unit ID |
| Port | COM port | TCP 502 |
| Speed | 9600-115200 baud | 10/100/1000 Mbps |
| Max Distance | 1200m (9600 baud) | 100m per segment (Ethernet) |
| Masters | One per bus | Multiple simultaneous |
When to Choose RTU
- Field devices on RS-485 buses
- Legacy instruments
- Remote sites with serial radios
- Low-cost sensor networks
When to Choose TCP
- Ethernet-connected devices
- SCADA and DCIM integration
- Multi-master access needed
- High-speed requirements
See RS-485 wiring guide and TCP network setup for implementation details.
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Frequently Asked Questions
Yes. 100Mbps Ethernet is significantly faster than serial RS-485 (max 115200 bps). However, actual response time depends on network latency and master polling design.
Yes. The data model — register types and addresses — is identical between variants.
Still have questions about your setup? Talk to an engineer or call (210) 405-4248.