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by anonymous

I'm trying to use RS485 for communication with PLC because the PLC does not yet support modbusTCP (Beckhoff CX7080).
i'm using RUT...06.09. version.

I tried using under services -> RS232/RS485 the RS485 settings but when i started modbuspool and connected i saw in PLC (slave) error: "Illegal Function". I'm presuming, that "RS232/RS485" is not Modbus but only RS485 protocol. Is that correct or how?

I have also tried using Services -> Modbus and then Serial Master but can't find how to use the further.. so how to read reg.

My PC is connected to RUT LAN port. and on RS485 connector is reciver and driver (-) connected together and also reciver and driver (+) together.

I have tried with a master PLC and TGW715 and know that the "slave" works OK.

Can anyone explain what is wrong or has some correct instructions on how to do this?

1 Answer

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by anonymous


In Over IP mode, as is set in the provided screenshot, the router acts as a transparent gateway, independent on the type/format/content of the data, forwarding data between client and server devices, thus communication must be initiated from the client side device/application. 

If PLC does use MODBUS, you should configure your device as follows:

  • First, to simulate PLC, I have used ModRSsim2 application to set some registers with random values:


  • Next follows router's RS485 configuration:
    • Enabled: Yes;
    • Baud rate: 115200 (set this with the value matching your PLC configuration);
    • Parity: None;
    • Flow control: None;
    • Serial type: Modbus gateway;
    • Listening IP: (router's LAN IP);
    • Port: 502 (standard, well known port);
    • Slave ID configuration type: User defined;
    • Slave ID: 1.

  • Lastly, to read register values I have used QModMaster application with the following Modbus TCP settings:
    • Slave IP:;
    • TCP port: 502.

  • This allowed to read Modbus registers from the simulator application:

Best regards,

Best answer
by anonymous
Today i have tried your option and it worked. I have tested it with Beckhoff CX8180 PLC as RTU Slave and could read registors.

So in case of connecting over WAN IP on VPN Connection (OpenVPN) i just input in firewall port forward 502 to RUTs internal IP or does it need to be configured differently.