Context: I am attempting to connect a series of remote Siemens PLCs via OpenVPN, using the RMS Remote Software Solution provided by Teltonika Networks. The VPN hub is set up correctly, and the PLC routes are forwarded through the router connected to the VPN hub. The local IP addresses and subnet masks are configured properly. The Modbus server is hosted on the router itself. The Siemens PLC is intended to detect this router as a local device and communicate using the MB_CLIENTfunction block.

Problem: The PLC intermittently throws the error #8384 and cycles between errors #7004, #7005, and #7006. The data pointer remains empty, indicating that no data is being received. I have tested the connection directly from my PC using the VPN IP, and the Modbus server responds correctly, so the issue seems to be specific to the PLC configuration.

Configuration:

• VPN Hub: OpenVPN provided by RMS Remote Software Solution from Teltonika Networks.
• Router IP: Assigned locally by the VPN.
• Modbus Server: Hosted on the router.
• PLC Function Block:MB_CLIENTused to connect to the Modbus server.

Detailed Configuration Steps:

1. VPN Connection:

   • Successfully connect to the VPN hub using OpenVPN.
   • Forward PLC routes over the router connected to the VPN hub.

2. Modbus Server Configuration:

   • The Modbus server is hosted on the router.
   • The router's IP and subnet mask are configured to appear as a local device to the PLC.

3. Siemens PLC Configuration:

   • MB_CLIENTfunction block is used to connect to the Modbus server.
   • IP address and parameters are set correctly, both in hexadecimal and decimal formats.

Symptoms:

• The PLC occasionally reports error #8384.
• Cycles between errors #7004, #7005, and #7006.
• Data pointer remains empty despite successful connection tests directly from a PC.

Steps Taken:

1. Verified the connection by directly accessing the Modbus server from a PC using the VPN IP address.
2. Ensured that all parameters in theMB_CLIENTfunction block are correctly set.
3. Tried different formats (hexadecimal and decimal) for setting the IP address and parameters.

Request for Assistance:

1. Verification:

   • Are there specific settings or best practices for configuring theMB_CLIENTfunction block for use with VPN-connected devices?
   • Could there be additional settings or network configurations required for stable communication over OpenVPN?

2. Error Handling:

   • What are the typical causes of error  #8384 in the context ofMB_CLIENTfunction block?
   • Any recommended approaches to diagnose and resolve these errors?

3. Connectivity:

   • Could the issue be related to VPN handling of Modbus TCP traffic? Any specific configurations needed on the router or VPN settings?
   • Are there known issues or limitations with using theMB_CLIENTfunction block over a VPN connection?

Any insights, suggestions, or troubleshooting steps would be greatly appreciated.