Adding Profibus capabilities to a product
Following this link you will find profibus software implementations in assembler (AVR) and C (AVR and MSP). Speed is up to 187.5kBps, but using XMEGAs I guess that few higher speeds would also be possible. beware that with software only implementation and without being a member, it will be hard to certify your product officially (which may or may not be important to you).

If certification does matter, then it's easiest to use some already certified Profibus chip. At this link you will find AVR and 8051 example interfacing VPC3+C Profibus chip.

You can download GSD editor here.

For testing, there was free PC software (if you can't google it then let me know and I will check my notes) that allows you to do the testing even with 9.6 and 19.2 Kpbs, which means that you don't even need any special card (except RS232&lt;>RS485 convertor), since these are standard speeds available on PC. For more serious testing you need to have a professional equipment like Procentec ProfiTrace. It is very powerful but very expensive.

If you need consulting services, then let me know with a private message.