Needs["NETLink`"]
InstallNET[];
ser=NETNew["System.IO.Ports.SerialPort","COM4",9600]
ser@Open[]
Button["-1",ser@Write["A"]]

Button["-2",ser@Write["B"]]
Button["One More?",ser@Write["C"]]
Button["stop program",ser@close[]]

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