now I had time to check my serial port with the Serial-Monitor.
I could see that one single byte come out of the serial port, but nothing more.
Is it possible that the serial port in DosBox is only able to send data, but unable to receive data?
However, I think there is no solution for my problem. I read in other borads, that DosBox has some problems with the serial port. So I think the best is to install Win98 on by computer parallel to XP.
Many thanks for your help!