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Modbus RTU vs Modbus TCP: what is the difference?

Modbus RTU runs on serial RS-485 and Modbus TCP runs on Ethernet. Compare wiring, speed, addressing and when to use each.

  • 5 min read
  • Updated

Side by side

  • Physical layer: RTU uses RS-485 (or RS-232) serial wiring. TCP uses standard Ethernet.
  • Topology: RTU is a daisy-chain bus with one master and up to 247 slave addresses. TCP is a client-server network where many clients can connect.
  • Speed: RTU typically runs from 9,600 to 115,200 baud. TCP runs at Ethernet speed.
  • Data model: both use the same coils, discrete inputs, input registers and holding registers.
  • Error checking: RTU adds a CRC to each frame. TCP relies on the TCP/IP checksum.

  • Use twisted-pair, shielded cable and daisy-chain the devices.
  • Terminate both ends of the bus with the correct resistor, and bias the line where needed.
  • Give every device a unique address and the same baud rate, parity and stop bits.

A Modbus gateway can connect RTU devices to a TCP network. This lets a PLC or SCADA system on Ethernet read serial meters and drives without extra cabling to each one.

Frequently asked questions

Which Modbus is faster?

Modbus TCP, because it runs on Ethernet. RTU is limited by its serial baud rate.

What is the TCP port for Modbus?

Modbus TCP uses TCP port 502 by default.

Is Modbus secure?

The basic protocol has no built-in authentication or encryption, so keep Modbus on a protected network or behind a firewall.

This guide is general engineering guidance. Always confirm part-specific details, ratings and compatibility against the manufacturer’s datasheet. We confirm price, stock and specifications for the exact part in a written quotation. See the glossary for the terms used here.