Write to Serial port

The project am working on involves reading inputs from the serial port which I did using the System.IO.Ports. Now, I want Unity to write my Y-axis to an arduino chip. (I want to be able to detect hills on my terrain and use the information) How do I go about that, and can I use the same COM port that I'm already using to read?

Did you try searching for 'serial port' on this forum? In short, yes.