Siemens S7-1200 System Manual page 734

Communication processor and Modbus TCP
12.5 Modbus 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 511). 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
connection.
Table 12- 52 MB_CLIENT instance data block user accessible static variables
Variable
Blocked_Proc_Timeout
MB_Unit_ID
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data type.
Data type Default description
Real 3.0 Amount of time (in seconds) to wait upon a blocked Modbus client
instance before removing this instance as being ACTIVE. This can
occur, for example, when a client request has been issued and then
application stops executing the client function before it has completely
finished the request. The maximum S7-1200 limit is 55 seconds.
Word 255 Modbus unit identifier:
A Modbus TCP server is addressed using its IP address. As a result,
the MB_UNIT_ID parameter is not used for Modbus TCP addressing.
The MB_UNIT_ID parameter corresponds to the slave address in the
Modbus RTU protocol. If a Modbus TCP server is used for a gateway to
a Modbus RTU protocol, the MB_UNIT_ID can be used to identify the
slave device connected on the serial network. The MB_UNIT_ID would
be used to forward the request to the correct Modbus RTU slave
address.
Some Modbus TCP devices may require the MB_UNIT_ID parameter to
be initialized within a restricted range of values.
S7-1200 Programmable controller
System Manual, 03/2014, A5E02486680-AG