Modbus RTU to Modbus TCP converter

I'm new to posting in forums and I'm also quite inexperienced in working with industrial communications, so I hope someone can help me out.
I have a USR-DR302 RS485 to ethernet converter, however, when I set up the device and try to read values in my computer from the slave device, I keep getting connection errors. I'm just trying to make sure that the converter is working as it's supposed to, but since I can't read any values I'm kind of stuck. To read the values I'm using Modbus tester and Modbus poll. For some context, I'm basically trying to do what this tutorial is doing but with a different converter, do you have any idea what could be going on? The device I'm trying to read data from is a VFD Delta MS300 but I have to read the data using the USR converter, since the VFD is not the final product I will be using it on.
Thank you very much


Using a USB to RS-485 adapter is very different from using an Ethernet to RS-485 adapter.

The problem here is that there are many different (and incompatible) versions of Modbus depending on whether RS-485 or Ethernet is used. There are two Modbus versions that run on RS-485: Modbus RTU and Modbus ASCII. Officially, there is only one Modbus version that runs on Ethernet: Modbus/TCP. Unofficially, there are several variants like Modbus RTU Over TCP, which essentially encapsulates the Modbus RTU packet in a TCP frame and sends it over Ethernet. Note that this is exactly what your Ethernet to RS-485 adapter is doing!

Modbus Poll does support a variety of Modbus variations, including Modbus RTU Over TCP. You can select the method in the Connection Setup window:

On the USR-DR302, I believe you need to select "TCP Server" for the "Module work mode" setting, since Modbus Poll is the TCP Client.

You need to also make sure both Modbus Poll and the USR-DR302 are configured to use the same TCP port (Server Port in Modbus Poll and I believe Local Port on the USR-DR302, since I would imagine Remote Port to only be used in TCP Client mode). And make sure that the Modbus Poll's Remote Modbus Server matches the IP address that the USR-DR302 is configured for.
1. converter's COM port

screenshot "Modbuspoll.png"

I doubt that Window's Device Manager will report your USB/RS-485 converter's port as a "Serial over Bluetooth link". Pick a port associated with USB device.

2. Any reason you don't use the standard Modbus TCP/IP port 502?

With Windows security lockdowns, could your firewall be blocking port 8234?