Needs["NETLink`"]
InstallNet[];
ser=NETNew["System.IO.Ports.SerialPort","COM8",9600]
ser@Open[]

{
Button["",ser@Write["A"],Background->Green,ImageSize->{50,50}]
Button["",ser@Write["B"],Background->Yellow,ImageSize->{50,50}]
Button["",ser@Write["C"],Background->Red,ImageSize->{50,50}]
}
{
Button["",ser@Write["D"],Background->Green,ImageSize->{50,50}]
Button["",ser@Write["E"],Background->Yellow,ImageSize->{50,50}]
Button["",ser@Write["F"],Background->Red,ImageSize->{50,50}]
}
{
Button["",ser@Write["G"],Background->Green,ImageSize->{50,50}]
Button["",ser@Write["H"],Background->Yellow,ImageSize->{50,50}]
Button["",ser@Write["I"],Background->Red,ImageSize->{50,50}]
}

Button["Stop Program",ser@Close[]]