Siemens SIMATIC S7-1200 System Manual page 1254

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Communication processor and Modbus TCP

13.8 Legacy Modbus TCP communication

● Complex DB elements can be assigned by MB_DATA_PTR, such as

– Standard arrays

– Named structures where each element is unique.

– Named complex structures where each element has a unique name and a 16 or 32 bit

● There is no requirement that the MB_DATA_PTR data areas be in the same global data

block (or M memory area). You can assign one data block for Modbus reads, another

data block for Modbus writes, or one data block for each MB_CLIENT station.

Multiple client connections

A Modbus TCP client can support concurrent connections up to the maximum number of

Open User Communications connections allowed by the PLC. The total number of

connections for a PLC, including Modbus TCP Clients and Servers, must not exceed the

maximum number of supported Open User Communications connections (Page 801). The

Modbus TCP connections may be shared between Client and/or Server type connections.

Individual client connections must follow these rules:

● Each MB_CLIENT connection must use a distinct instance DB

● Each MB_CLIENT connection must specify a unique server IP address

● Each MB_CLIENT connection must specify a unique connection ID

● Unique IP port numbers may or may not be required depending upon the server

configuration

The Connection ID must be unique for each individual connection. This means a single,

unique Connection ID must only be used with each individual instance DB. In summary, the

instance DB and the Connection ID are paired together and must be unique for every

S7-1200 Programmable controller

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