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

Button["Stop Program",ser@Close[]]
Button["",ser@Write["A"],Background->Red,ImageSize->{50,50}]
Button["",ser@Write["B"],Background->Green,ImageSize->{50,50}]
Button["",ser@Write["C"],Background->Yellow,ImageSize->{50,50}]