Needs["NETLink`"]
InstallNET[];

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

ser=NETNew["System.IO.Ports.SerialPort","COM8",9600]
ser@Open[]

«NETObject[System.IO.Ports.SerialPort]»

Button["START",ser@Write["S"]]
Button["A",ser@Write["A"]]
Button["B",ser@Write["B"]]
Button["C",ser@Write["C"]]
Button["D",ser@Write["D"]]
Button["E",ser@Write["E"]]
Button["STOP",ser@Close[]]