Siemens SIMATIC S7-1200 System Manual page 1177

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● RemotePort: The default is 502. This number is the IP port number of the Modbus server

that MB_CLIENT attempts to connect and communicate with. Some third-party Modbus

servers require that you use another port number.

● LocalPort: This value must be 0, for an MB_CLIENT connection.

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).

Individual concurrent client connections must follow these rules:

● Each MB_CLIENT connection must use a unique instance DB

● Each MB_CLIENT connection must assign a unique server IP address

● Each MB_CLIENT connection must assign a unique connection ID

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

configuration

A different connection ID must be used with each instance DB. In summary, the instance DB

and the connection ID are paired together and must be unique for each connection.

Table 13- 59 MB_CLIENT instance data block: User accessible static variables

Blocked_Proc_Timeo

S7-1200 Programmable controller

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

Amount of time (in seconds) to wait upon a blocked Modbus client in-

stance 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 completely finishing the re-

quest. The maximum S7-1200 limit is 55 seconds.

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 within a restricted range.

Time in seconds that the MB_CLIENT waits for a server to respond to a

Indicates whether the connection to the assigned server is connected or

disconnected: 1=connected, 0=disconnected

Communication processor and Modbus TCP

13.5 Modbus communication

Siemens SIMATIC S7-1200 System Manual page 1177

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