S.Port data transfer via ATMEGA23u4 via softwareserial

I want to use an ATMEGA32u4-Beetle sensor to transmit data to a FrSky 
receiver via the S.Port interface. I use the Softwareserial library to 
invert the data. The High-TTL level of the receiver (see picture) is 4V, 
but the High-TTL level of the ATMEGA32u4 is only 2V. I have also tried 
various resistors between 5V and/or directly in the S wire but without 
success. What am I doing wrong?