Hello,

problematic is generally:

1. for first media conversion - Ethernet vs Serial.

2. conversion of Protocol, in this case Modbus TCP to Modbus RTU or Modbus ASCII, all depends to exact use of your Slave protocol. Check it carefully.

3. RS-232 allow only one Partner connected. RS-485 allow basically more Slaves on bus. Modbus can be used on RS-232 and also on RS-485.

You have already some sugestion but you need be carefully here, especially with that adapters. Basically direct connection is impossible, you mix different media for data exchange. This isn't strictly Modbus problematic but general in communication conversion.