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Modbus RTU vs. Modbus TCP: Differences, Wiring and Common Pitfalls
Modbus is the most widely supported industrial protocol: almost every PLC, drive, meter and HMI speaks it. Modbus RTU runs over serial lines, usually RS-485. Modbus TCP runs over Ethernet. They use the same data model and function codes, so moving between them is mostly about the physical connection and a few settings.
Comparison
| Modbus RTU | Modbus TCP | |
|---|---|---|
| Physical layer | RS-485 (multi-drop) or RS-232 (point-to-point) | Ethernet |
| Addressing | Slave/server ID 1–247 | IP address, TCP port 502, plus a unit ID |
| Speed | Typically 9,600–115,200 baud | 10/100 Mbit Ethernet |
| Error checking | CRC on every message | Handled by TCP/IP |
| Masters/clients | One master on the bus | Multiple clients can talk to one server |
| Cabling | Twisted pair daisy chain, up to about 1,200 m at low baud rates | Standard Ethernet cable and switches, 100 m per segment |
What they share: the data model
| Type | Traditional address | Size | Access |
|---|---|---|---|
| Coils | 0xxxx | 1 bit | Read/write |
| Discrete inputs | 1xxxx | 1 bit | Read only |
| Input registers | 3xxxx | 16 bits | Read only |
| Holding registers | 4xxxx | 16 bits | Read/write |
Common function codes: 01 read coils, 02 read discrete inputs, 03 read holding registers, 04 read input registers, 05/15 write coils, 06/16 write registers.
RS-485 wiring rules for Modbus RTU
- Daisy-chain devices one after another. Avoid star wiring and long stubs.
- Use shielded twisted pair for the A/B (D+/D–) signals and connect the signal common.
- Fit a 120 Ω termination resistor at each end of the bus, and only at the ends.
- Make sure the bus has biasing (usually provided by the master or a switch on one device).
- Every device needs a unique address, and the same baud rate, parity and stop bits.
- Watch A/B labeling: manufacturers disagree on which is which. If nothing responds, try swapping them.
The most common pitfalls
- Off-by-one addresses. "40001" in a manual usually means holding register offset 0 on the wire. Some software wants 40001, some wants 0, some wants 1. If every value is shifted by one register, this is why.
- Byte and word order. 32-bit values (floats, long integers) use two registers, and devices disagree on their order. If a value looks wildly wrong, swap the words.
- Scaling. Many devices send integers scaled by 10 or 100 (e.g. 2345 = 234.5 V). Check the register map.
- Timeouts and polling too fast on RS-485 networks with many devices.
Which should you choose?
- Modbus TCP for new installations with Ethernet available: easier diagnostics, faster, and many clients can share a device.
- Modbus RTU for simple, low-cost links to drives, meters and instruments that only have RS-485, or for long cable runs.
- Gateways convert between them, and between Modbus and other protocols. Unitronics offers gateways such as the GW-BAC1 (BACnet to Modbus) and GW-KNX1 (KNX to Modbus).
Unitronics controllers, including UniStream panels such as the USP-104-C10, support Modbus communication; check the model's specifications for ports and protocols. Comparing other industrial Ethernet protocols? See EtherNet/IP vs. PROFINET vs. EtherCAT.
Frequently asked questions
Is Modbus TCP the same as Modbus over Ethernet?
Yes, in common use.
Can Modbus RTU and Modbus TCP devices talk directly?
Not without a gateway, because the physical layers differ.
Need hardware?
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