but at same time (well no really) I have to check the radio-modem if there is data available

Ok then you want the interupt on your Radio. So that it will take over the keyboard. (since the keyboard is not as important)

Mister-e' idea of a interupt and USART should do the trick. The interput will immediately switch to the radio, and receive the data. At the "End of Transmittion" it will finish displaying and return you to the keyboard. The only thing that will happen, is the keyboard may loose a character or you may have to retype the very last character.

Dwayne