Ho copiato e incollato l'errore....

France_CE.exe
IOException
in System.IO.Ports.SerialStream.WinIOError()
in System.IO.Ports.SerialStream..ctor()
in System.IO.Ports.SerialPort.Open()
in France_CE.ModBus.openComunication()
in France_CE.Form1..ctor()
in France_CE.Bunner..ctor()
in France_CE.Program.Main()