Hello, 

im working on a project where i need to communicate with a LOGO via Modbus RTU. For the communication I have a CIM that will take the modbus RTU and use S7 to write the values directily to the logo. This works when no other devices are on the Modbus net. It also works if the LOGO is not connected to the CIM. 

Every time I try it with all devices plugged in I get an timeout error and the CIM becomes unresponsive. I then have to completely reboot the system to have it working again. Also the problem doesn't occour if I only transmit zeros, aka no change.

I have tried the communication in various ways, like Modbus TCP instead of S7 or different slave/master configurations. But always the same problem. I have no devices with the same address on the Modbus RTU net and changed it multiple times. Also my cable runs are very short. I can read all other sensors with no problem.

I also changed the timeout to over a second and I only want to set some coils/bits so nothing big.

Has anybody the same problem or knows a way around it? Is there a better way to do this?

Thanks in advance!

Hi AutoGen_7787001

This issue sounds familiar to me.

My config: CIM as router with RS485 RTU device connected, 2 Logo8. 3 server connections with green checked connection. Diag LED lits green. No data tansfer. Writing a 4th fake server connection re-establish the connection. Seems settings are gone but not visible at login with a browser, everything looks fine then.

In the https://support.industry.siemens.com/cs/document/109806055/logo!-cim-modbus-rtu-communication-with-sinamics-v20?dti=0&lc=en-NL two devices connected to the modbus RTU.

Hi,

A CIM can only communicate to one LOGO! 8. But you can connect several Modbus RTU devices to one CIM. In this case each ModbusRTU device must have a different unit ID.

I do not have much experiances with ModbusRTU sensors but you description is a hint to me that your ModbusRTU devices uses the same Unit-ID.

The picture shows where you need to set the different Unit-ID of your ModbusRTU sensors. In addition normaly if you get two sensors from the same vendor, than they have the same Unit-ID and you need to change/configure it.

Hope this helps

Hey, the thing is, everything works, but as soon as I plug in any other modbus RTU device, I get into trouble. Also sending zeros works, but nothing else. 

The picture only showed how to forward the input values or something else?

Thank you!

Hi,

If you have one ModbusRTU device with "default" unit-ID running and you connect another one, than you have two modbusRTU devices connected to CIM with the same unit-ID.

You need to change the unit-ID first to not have two devices with the same unit-ID connected to CIM.

Hope this helps