Siemens SIMATIC S7-1200 System Manual page 1187

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Example: MB_CLIENT 1: Multiple requests with common TCP connection

Multiple Modbus client requests can be sent over the same connection. To accomplish this,

use the same instance DB, connection ID, and port number.

Because both MB_CLIENT boxes use the same CONNECT parameter TCON_IP_v4 data

structure ("Modbus_connections".TCPactive_1), the connection ID, port number, and IP

address are identical. The CONNECT IP address data assigns the IP address, of the target

Modbus TCP server.

Only one MB_CLIENT can be active at any given time. Once a client completes its

execution, the next client can begin execution. Your program logic is responsible for the

execution sequence logic. The example shows both clients reading remote data from a

single Modbus client and transferring the data to the Modbus client's CPU (M memory

starting at M1000.0). A returned error is captured, which is optional.

Network 1: Modbus function 1 - Read 16 output bits from a Modbus TCP server with the IP

address assigned in "Modbus connections".TCPactive_1.

Network 2: Modbus function 2 - Read 32 input bits from a Modbus TCP server with the IP

address assigned in "Modbus connections".TCPactive_1.

S7-1200 Programmable controller

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

Communication processor and Modbus TCP

13.5 Modbus communication