Hi
thanks for the reply.

the code below (I have stripped out some lines )shows what I am trying to do. It crashes with any code in OnSerialDataReceived even if its just updating a text box. I can see the data if i step through the code in OnSerialDataReceived so that part works.

sorry for the long delay I have had one of those weeks :)
any help gratefully received
thanks
Stuart

      public SerialPort ComPort;
       public scan1 oScan;

             oScan = new scan1();
                
              ComPort = new SerialPort(port, baud);
                      
                ComPort.Parity = Parity.None;
                ComPort.StopBits = StopBits.One;
                ComPort.DataBits = 8;
                ComPort.Handshake = Handshake.None;
                ComPort.DataReceived += OnSerialDataReceived;
      
        public void OnSerialDataReceived(object sender, SerialDataReceivedEventArgs args)
        {

            string data = this.ComPort.ReadExisting();
         
            oScan.updateScan(data);

        }