And RS232 is inverted compared to TTL signals, so you need something like a max232 to convert rs-232 to ttl and maybe a logic port to invert it then, or take the atmega328p out of the arduino and use the digital 0 and 1 because they are connected to the ftdi chip that is in fact an usb-ttl serial converter, only if you use the duemilanove or older, the newer Uno uses an atmega32u2 that does the usb-serial convertion, but it migh work as well.