RS232 relay control under OSX

I want to write a program that will allow me to control some external electronic relays connected to my macbook via a USB/Rs232 (serial/TTY) connection. In windows I was able to set individual pins on the RS232 to high or low from within my program (in something similar to this) I was wondering if there was anyway of doing this using objective c under osx? If anyone has seen any guides to diy hardware control under osx, that would be useful to, cheers