Siemens SIMATIC S7-1200 System Manual page 1168

Hide thumbs Also See for SIMATIC S7-1200:

Table of Contents

Communication processor and Modbus TCP

13.5 Modbus communication

Modbus TCP communication

Modbus TCP (Transmission Control Protocol) is a standard network communication protocol

that uses the PROFINET connector on the CPU for TCP/IP communication. No additional

communication hardware module is required.

Modbus TCP uses Open User Communications (OUC) connections as a Modbus

communication path. Multiple client-server connections may exist, in addition to the

connection between STEP 7 and the CPU. Mixed client and server connections are

supported up to the maximum number of connections allowed by the CPU model

Each MB_SERVER connection must use a unique instance DB and IP port number. Only 1

connection per IP port is supported. Each MB_SERVER (with its unique instance DB and IP

port) must be executed individually for each connection.

A Modbus TCP client (master) must control the client-server connection with the

DISCONNECT parameter. The basic Modbus client actions are shown below.

1. Initiate a connection to a particular server (slave) IP address and IP port number

2. Initiate client transmission of a Modbus message and receive the server responses

3. When desired, initiate the disconnection of client and server to enable connection with a

different server.

Modbus RTU instructions in your program

● Modbus_Comm_Load: One execution of Modbus_Comm_Load is used to set up PtP port

parameters like baud rate, parity, and flow control. After a CPU port is configured for the

Modbus RTU protocol, it can only be used by either the Modbus_Master or

Modbus_Slave instructions.

● Modbus_Master: The Modbus_Master instruction enables the CPU to act as a

Modbus RTU master device and communicate with one or more Modbus slave devices.

● Modbus_Slave: The Modbus_Slave instruction enables the CPU to act as a Modbus RTU

slave device and communicate with a Modbus master device.

Modbus TCP instructions in your program

● MB_CLIENT: Make client-server TCP connection, send command message, receive

response, and control the disconnection from the server

● MB_SERVER: Connect to a Modbus TCP client upon request, receive Modbus message,

and send response

S7-1200 Programmable controller

System Manual, V4.2, 09/2016, A5E02486680-AK