Siemens S7-1200 System Manual page 738

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Communication processor and Modbus TCP

12.5 Modbus communication

The following table shows examples of Modbus address to holding register mapping used for

Modbus function codes 03 (read words), 06 (write word), and 16 (write words). The actual

upper limit of DB addresses is determined by the maximum work memory limit and M

memory limit, for each CPU model.

Table 12- 58 Mapping examples of Modbus address to CPU memory address

Modbus Address

Multiple server connections

Multiple server connections may be created. This permits a single PLC to establish

concurrent connections to multiple Modbus TCP clients.

A Modbus TCP server 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 511). The

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

Individual server connection must follow these rules:

● Each MB_SERVER connection must use a distinct instance DB.

● Each MB_SERVER connection must be established with a unique IP port number. Only 1

connection per port is supported.

● Each MB_SERVER connection must use a unique connection ID.

● The MB_SERVER must be called individually for each connection (with its respective

instance DB).

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

P#M100.0 Word 5

MB_HOLD_REG parameter examples

P#DB10.DBx0.0 Word 5

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S7-1200 Programmable controller

System Manual, 03/2014, A5E02486680-AG