Legacy Modbus Rtu Examples; Example: Legacy Modbus Rtu Master Program - Siemens SIMATIC S7-1200 System Manual

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Legacy Modbus RTU examples

MB_COMM_LOAD is initialized during start-up by using the first scan flag. Execution of

MB_COMM_LOAD in this manner should only be done when the serial port configuration will

not change at runtime.

Network 1: Configure/initialize the RS485 module communications port only once during the

One MB_MASTER instruction is used in the program cycle OB to communicate with a single

slave. Additional MB_MASTER instructions can be used in the program cycle OB to

communicate with other slaves, or one MB_MASTER FB could be re-used to communicate

with additional slaves.

Network 2: Read 100 words of holding register data from location 400001 on slave #2 to

memory location MW500-MW698.

S7-1200 Programmable controller

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

Communication processor and Modbus TCP

13.9 Legacy Modbus RTU communication (CM/CB 1241 only)