Hi everyone
am trying to establish Modbus RTU communication between siemens CM1241 (485/422) module and delta VFD M-Serie (with Rj-11), the picture shows how i did the connection, but it does not works, so i switched (3 to 3 and 4 to 8) and for the CM1241 GND i used pin 1 and pin 5 ,so with all it does not work
both plc and vfd uses the same GND
where is the problem !!!

 

Modbus RTU communication issues almost always come down to either wiring or configuration settings.

Your wiring diagram appears correct, so I believe the issue is configuration settings.

Of course, the first thing to confirm is that both devices are configured for the same baud rate and parity, and that the Siemens PLC is targeting the same Modbus slave address that the VFD is configured for.

Since the CM1241 supports both RS-485 and RS-422, you will need to make sure that you've selected RS-485. According to this (https://support.industry.siemens.co...communication-from-s7-1200-plc-cm-1241/149654) it seems there should be a MODE setting in the MB_COMM_LOAD block, however, your image shows a MODE setting on the MB_MASTER block, so perhaps the setting is there instead.

Also, the VFD supports multiple Modbus encodings (both ASCII and RTU), so you need to make sure that you have Modbus RTU selected as the protocol (Pr. 92, the default is Modbus ASCII). Note you may need to power cycle the VFD for any setting changes to take effect.

 

I followed those instruction https://plc247.com/siemens-s7-1200-modbus-rtu-delta-vfd-m-tutorial/
and i did many changes in both plc and vfd to any effects but steal not working
i changed the modbus types and the start adresses and many ather parameters but no resaults
something to mention is that when i activate one of the MB_master the Tx keep blinking

 

I recommend testing each piece of equipment individually using a computer and a USB to RS-485 adapter to determine where the problem lies. There are several Modbus software tools available, for example:

WinTECH ModScan and ModSim
http://win-tech.com/

Simply Modbus master and slave simulators
http://www.simplymodbus.ca/

Witte Software Modbus Poll and Modbus Slave
https://www.modbustools.com/download.html

All of the above are paid software, but have demo versions available that should be sufficient for testing purposes.