Thanks for your response.
The first option that you suggested, we checked that our communication cable is separate from the power wiring.
and in the second option, we can not change the baud rate of s7-300 PLC.
The baud rate of PLC and HMI is 187.5kbps and will not be changeable.
communication cable connection is:
DOP------MPI
1--------3
6-------8
5-------5