Needs["NETLink`"]
InstallNet[];

NETTypeInfo["System.IO.Ports.SerialPort"]

ser=NETNew["System.IO.Ports.SerialPort","COM8",9600]
ser@Open[]
Button["CONNECT",ser@Open[]]
Button["BREAK CONNECTION",ser@Close[]]
{Slider[Dynamic[x],{0,100,1}],Dynamic[x],Dynamic[Refresh[ser@Write[MakeNETObject[{x,x},"System.Byte[]"],0,2],UpdateInterval->0.1];"LEDBAR"]}