MODBUS-TCP

Modbus encapsulated in TCP port 502.

Definition

Modbus TCP structure: MBAP Header (Transaction ID 2 bytes, Protocol ID 0x0000 2 bytes, Length 2 bytes, Unit ID 1 byte) + Modbus PDU (Function Code 1 byte + Data N bytes). Main Function Codes: 01 Read Coils, 02 Read Discrete Inputs, 03 Read Holding Registers, 04 Read Input Registers, 05 Write Single Coil, 06 Write Single Register, 15 Write Multiple Coils, 16 Write Multiple Registers, 23 Read/Write Multiple Registers. Massive adoption: Schneider PLCs, Siemens Wincc Web, AB Allen Bradley, Mitsubishi, ABB.

Origin

Modbus protocol created 1979 by Modicon (acquired by Schneider Electric 1996) ; Modbus TCP published 1999 ; standardised IEC 61158 type 15 published 2007.

Example in context

A 5MW photovoltaic energy plant uses Modbus TCP to read 200 SMA Sunny inverters ; SCADA polls inverters via Function Code 03 (Read Holding Registers) every 5 seconds.

Related terms

• Modbus RTU — RS-485 serial counterpart.