Modbus TCP vs RTU

Tutorials › Modbus TCP vs RTU

Modbus comes in two variants you will meet constantly in the field: Modbus RTU over serial and Modbus TCP/IP over Ethernet. They carry the same data model - coils, discrete inputs, input registers and holding registers - and the same function codes. What differs is the transport: how the message is framed and physically moved between devices.

The data is the same, the transport is not

Both variants use the same Protocol Data Unit (PDU): a function code plus data. A master reads or writes the same registers either way. So if you can talk to a device over RTU, the register map is identical over TCP - only the connection changes.

Frame format

Modbus RTU wraps the PDU with a 1-byte slave address at the front and a 2-byte CRC checksum at the end. Message boundaries are detected by silent gaps on the wire (about 3.5 character times).

Modbus TCP drops the CRC (TCP/IP already guarantees integrity) and adds a 7-byte MBAP header containing a transaction ID, protocol ID, length and unit ID. The unit ID replaces the RTU slave address and is often used when bridging to serial devices through a gateway.

Wiring

RTU almost always runs on RS-485, a two-wire (plus common) differential bus that lets many devices share one pair of wires over long distances. Terminate both ends with 120 Ω resistors and keep a consistent A/B polarity. TCP runs on ordinary Ethernet cabling to a switch - simpler to wire, but each device needs an IP address.

Which should you use?

• Choose RTU for simple, cost-sensitive, long-distance runs with a handful of serial devices - meters, drives and sensors on an RS-485 trunk.
• Choose TCP when you already have an Ethernet network, need higher throughput, want multiple clients, or are integrating with SCADA and IT systems.
• Mixed sites are common: a Modbus TCP-to-RTU gateway lets an Ethernet master reach serial devices using the unit ID.