Communication processor and Modbus TCP
12.5 Modbus communication
S7-1200 Programmable controller
738 System Manual, 03/2014, A5E02486680-AG
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
MB_HOLD_REG parameter examples
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
connection.
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