Arduino Project: FrSky S.Port (Smart.Port) data transfer with Beetle ATMEGA23u4 via softwareserial

I have developed a telemetry module for FrSky Smart.Port that works 
without any problems. As shown in the attachment I use an ATMEGA Beetle 
for the telemetry data transmission which is connected to the port D11 
and the S.Port on the RX8R.

The serial data transfer on the Beetle is done with the SoftwareSerial 
library with signal inversion.

What I cannot explain is why the TTL levels of the two communication 
partners are so different. The TTL level from the receiver has a little 
over 3V peak and the Beetle is also coming in at 1.67V.

I have no clue why that is so!

What can I do about it, for example, use a transceiver ...?