So it's ok, it works !
For those who want to drive external stuff with usb and relays, I recommend
- kmtronic products (I used http://www.sigma-shop.com/product/75/usb-4-relay-board-rs232-serial-controlled-pcb.html
- this page http://hallard.wikidot.com/synomodules
that I followed to build the drivers. Thanks to Charles-Henri Hallard.
I would add a few comments to avoid some difficulties I encountered (but I'm Ubuntu newbie) :
install ncurse with "apt-get install ncurses-dev"
use "make arch/powerpc/lib/crtsavres.o" to avoid a file not found during "make modules"
use make -i as we just need the drivers and don't care if any problem elsewhere.
As a conclusion, I would appreciate if someone is able to explain me how this set of commands
mknod /dev/ttyUSB0 c 188 0
echo -e "\xFF\x01\x01" > /dev/ttyUSB0 to be received by DS209II+ front port. Is it because "0" is its number ?