What fuse settings for Mega16U2

I have problems USB coprocessor Mega16U2 on an Mega2560. 2560 seems work. But programming with Arduino does not work. USB seems to work. So I think I'll check all fuse settings.

USB seems to work.

How do you know this?

But programming with Arduino does not work.

What indications do you get that it does not work? If you can't program the Arduino, how can you assume that either the USB chip or the Arduino are good?

Good questions.

I know that there is somethingg working in the USB chip, because now there is a COM port in the PC. And the devicelist shows something like 2560.

When I wrote a short test program/sketch, all went fine until IDE started to download that into the Arduino board. That download would newer finish