Siemens SIMATIC S7-1200 System Manual page 1182

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Communication processor and Modbus TCP

13.5 Modbus communication

The following table shows examples of Modbus addresses 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 13- 65 Mapping examples of Modbus address to CPU memory address

Modbus Address

Multiple server connections

Multiple server connections may be created. A single PLC can 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 801). The

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

Individual concurrent server connection must follow these rules:

● Each MB_SERVER connection must use a unique instance DB.

● Each MB_SERVER connection must assign a unique IP port number. Only 1 connection

per port is supported.

● Each MB_SERVER connection must assign a unique connection ID.

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

instance DB).

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