> Anyway, I can't switch it with stty command. What am I doing wrong?
> Thanks for your help.

Ages ago I also was not far from banging my head against a wall... 
There is nothing wrong, because the point is that /dev/tty0 is not openend any
more after the stty and then the kernel seems to reset the settings to the
default. Once you set the correct parameters in /etc/printcap for the serial
line, it will work. (Though I don't know if this restting of line parameters is
really what is intended in the kernel or just incidence). Perhaps a 'cat
/dev/tty0 &' will keep the line open and the settings intact... At least this
trick seems to work on IRIX 5.3, as I just verified.

Greetings from Paderborn...