U need RS232 Sniffer;-
DIY RS232 serial spy monitor cable
http://www.lammertbies.nl/comm/cable/RS-232-spy-monitor.html#full
RS232 Sniffer Passive Tap - USB Port
http://www.stratusengineering.com/EZTap.html
all u do is reverse engineering to make Arduino output same as PC.
electronics is art, not science.