Siemens SIMATIC S7-1200

Communication processor and Modbus TCP
13.8 Legacy Modbus TCP communication
S7-1200 Programmable controller
1266 System Manual, V4.2, 09/2016, A5E02486680-AK
13.8.4.3
Example: Legacy MB_CLIENT 2: Multiple requests with different TCP connections
Modbus client requests can be sent over different connections. To accomplish this, different
instance DBs, IP addresses, and connection IDs must be used.
The port number must be different if the connections are established to the same Modbus
server. If the connections are on different servers, there is no port number restriction.
The example shows both clients writing to the same memory area. Also, a returned error is
captured which is optional.
Network 1:
Modbus function 4 - Read input words (in S7-1200 memory)
Network 2:
Modbus function 3 - Read holding register words from a Modbus TCP server