Modbus Rtu Slave Example Program - Siemens S7-1200 System Manual

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Communication processor and Modbus TCP

12.5 Modbus communication

Modbus RTU slave example program

MB_COMM_LOAD shown below is initialized each time "Tag_1" is enabled.

Execution of MB_COMM_LOAD in this manner should only be done when the serial port

configuration will change at runtime, as a result of HMI configuration.

Network 1 Initialize the RS485 module parameters each time they are changed by an HMI

MB_SLAVE shown below is placed in a cyclic OB that is executed every 10ms. While this

does not give the absolute fastest response by the slave, it does provide good performance

at 9600 baud for short messages (20 bytes or less in the request).

Network 2 Check for Modbus master requests during each scan. The Modbus holding

S7-1200 Programmable controller

System Manual, 03/2014, A5E02486680-AG