SerialPort 问题

Question

            SerialPort port;
            port = new SerialPort();
            port.PortName = "COM3";
            port.BaudRate = 115200;
            port.StopBits = System.IO.Ports.StopBits.One;
            port.DataBits = 8;
            port.DtrEnable = true;
            port.RtsEnable = true;
            port.Parity = System.IO.Ports.Parity.None;
            port.Handshake = System.IO.Ports.Handshake.RequestToSendXOnXOff;
 
            if (port.IsOpen == false)
            {
                textBox1.Text += "open";
                port.Open();
                //到这里是正常的
            }

            //---------------这是发生异常  抛出System.IO.Ports.SerialStream.WinIOError()
            byte[] callCommand = Encoding.UTF8.GetBytes("at");
            port.WriteTimeout = 10000;//10S
            port.Write("at");
            //--------------
           
           
            textBox1.Text += "write";
            Thread.Sleep(500);
            string outputstring = port.ReadLine();
            textBox1.Text = outputstring;
            port.Close();
            port.Dispose();

环境： windows mobile 6.0   Smartphone

 //---------------这是发生异常  抛出System.IO.Ports.SerialStream.WinIOError()
            byte[] callCommand = Encoding.UTF8.GetBytes("at");
            port.WriteTimeout = 10000;//10S
            port.Write("at");
//-------------- 


补充      

          port.Open(); 之后 以下三个都只有读属性， 都为false
  
           // 获取端口的载波检测行的状态。
            textBox1.Text += "  CDHolding:" + port.CDHolding.ToString(); ;
            //获取“可以发送”行的状态。
            textBox1.Text += "  CtsHoldingToString:" + port.CtsHolding.ToString();
            //获取数据设置就绪 (DSR) 信号的状态。
            textBox1.Text += "  DsrHolding:" + port.DsrHolding.ToString();