Hi all,
thanks for your responses!
@MarkT: I hoped to find a way to avoid writing the interrupt driven code 
but it seems that I have to write such library (which will take more time for me than I thaught). My problem is that I have no scope, so I have to do trial & error for the signal generation (that's a problem with all other solutions, too).
@Bill_Kerr: Yes, that's also a possibility to avoid the software signal generation code. But I know that there is a working(!) software implementation based on UARTs (srcpd version 2 documentation). I will keep in mind to change to the hardware signal generation when everything will fail.
@skyjumper: I will use the the Arduino UNO (328P). It seems that someone has made a Port from OpenDCC to the Arduino (http://developer.berlios.de/projects/microsrcp/) which is also based on UART output (like srcpd). But I hope to get arround the tricky timings when using the "right" bit rate to communicate with.
It seems that I have to write my own signal generation code with interrupts or I have to use the hardware encoder from Motorola...
Regards, Holger