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The connection ID must be unique for each individual connection. A single, connection ID

must be used with each individual instance DB. The instance DB and the connection ID are

paired together and must be unique for every connection.

Table 13- 66 Modbus diagnostic function codes

MB_SERVER Modbus diagnostic functions

Sub-function

MB_SERVER variables

This table shows the public static variables that are stored in the MB_SERVER instance data

block and can be used in your program

Table 13- 67 MB_SERVER public static variables

HR_Start_Offset

Request_Count

Server_Message_Count

Xmt_Rcv_Count

Exception_Count

Success_Count

Your program can write data to the HR_Start_Offset and control Modbus server operations.

The other variables can be read to monitor Modbus status.

S7-1200 Programmable controller

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

Return query data echo test: The MB_SERVER will echo back to a Modbus client a data word

that is received.

Clear communication event counter: The MB_SEVER will clear the communication event counter

that is used for Modbus function 11.

Get communication event counter: The MB_SERVER uses an internal communication event

counter for recording the number of successful Modbus read and write requests that are sent to

the Modbus server. The counter does not increment on any request for Function 8, Function 11,

or any request that results in a communication error.

The broadcast function is not available for Modbus TCP, because only one client-server connec-

tion exists at any one time.

Data type Default

Communication processor and Modbus TCP

Assigns the starting address of the Modbus Holding register

The number of all requests received by this server.

The number of requests received for this specific server.

The number of transmissions or receptions that have encountered an

error. Also, incremented if a message is received that is an invalid

Modbus message.

Modbus specific errors that require a returned exception

The number of requests received for this specific server that has no

protocol errors.

Indicates whether the connection to the assigned client is connected or

disconnected: 1=connected, 0=disconnected

13.5 Modbus communication

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