Communicate with an IHM by port COM

Hi all,

I’m a newbie with beagleboard and i need help for my project.
I would communicate with an IHM by the port COM , i have a little electronic card with a port COM connected to my beagleboard black by the USB port.
In first i would switch off and on a led on the IHM for beginning, for make light the led i work in C language i have see examples on this site but not really what i need than this example

And i go work with linux on the beagleboard.

So to summarize i would switch on led on an IHM with a program in C in the beagleboard black with the help of port COM and i dont no how make that.