be80be:
You don't have the voltage level needed for that it's rs232c that's needs a 6 volt or higher swing you only have a 0 to 5 volt with this RS232 Shield For Arduino
you need a max 232 chip ±30-V Input Levels
It looks like the shield has a MAX232 chip and the MAX232 chip can generate the required RS232 voltages from 5V with its charge pump and external capacitors.