3.3V to 2.5V UART communication using Modbus

I need to connect my Arduino MKR1000 operating on 3.3 V to a sensor via UART. The sensors UART communicates on 2.5 V CMOS levels via Modbus ADUs, but using custom function codes. (For example the function code to write to the sensor's RAM is 0x41 and to read from the RAM is 0x44)

Do I need a level converter to communicate between the MKR1000 and the sensor? I checked the logic level voltages and from my understanding it should be working without, but I am not sure as I couldn't get a working communication yet.

Another problem I am having is to implement the Mobdbus communication in my code, because I don't know how to use the Modbus libraries with the custom functions my sensor has. Right now I have an array containing the ADU and send it using Serial.print(i) byte by byte. I am using the delay function to have the desired silence on the Serial port marking start and end.

Sadly my communication is not working at all. Do you have any suggestions on how to realize this?

What activity do you see on an oscilloscope?

The Tx pin just goes high when it wants to sends and stays high then. Same does the Rx pin when it's supposed to receive something.

Is this a continuation of your other thread at http://forum.arduino.cc/index.php?topic=441584.0 ?

Yeah, but I changed the code and noticed that the sensor is using a 2.5 V logic, so I figured I'd open a new thread with new information.