bool SendFlagLedOn = false;
           bool SendFlagLedOff = false;           

           Task recvTask = new Task(new Action(OnRxFrames));
            recvTask.Start();

            Task sendTask = new Task(new Action(OnTxFrames));
            sendTask.Start();

private void OnRxFrames()
        {
            while (true)
            {
                if (recvByteCnt == 0)
                {
                    recvLength = GetByteFromDevice();
                    recvByte = recvLength;
                }
                else
                {
                    recvByte = GetByteFromDevice();
                }

                recvBuffer[recvByteCnt] = recvByte;

                if (recvByteCnt ≥ recvLength)
                {
                    recvFlag = 1;
                    recvByteCnt = 0;

                    Debug.Write("Complete\n");

                    this.Invoke(new EventHandler(DoUpDate));
                }
                else
                {
                    recvByteCnt++;
                }
            }
        }

        private void OnTxFrames()
        {
            while (true)
            {
                if(SendFlagLedOn == true)
                {
                    SendFlagLedOn = false;

                    byte[] txData = new byte[5];
                    txData[0] = 4;
                    txData[1] = 1;
                    txData[2] = 2;
                    txData[3] = 3;
                    txData[4] = 4;

                    string str = Encoding.Default.GetString(txData);
                    mySerialPort.Write(str);
                }

                if (SendFlagLedOff == true)
                {
                    SendFlagLedOff = false;

                    byte[] txData = new byte[5];
                    txData[0] = 4;
                    txData[1] = 2;
                    txData[2] = 2;
                    txData[3] = 3;
                    txData[4] = 4;

                    string str = Encoding.Default.GetString(txData);
                    mySerialPort.Write(str);
                }
            }
        }

     fifo_peekonly = new ConcurrentQueue<byte>();
     fifo_queue = new BlockingCollection<byte>(fifo_peekonly);

        private void btn_Init_Click(object sender, EventArgs e)
        {
            mySerialPort = new SerialPort("COM5");

            mySerialPort.BaudRate = 115200;
            mySerialPort.Parity = Parity.None;
            mySerialPort.StopBits = StopBits.One;
            mySerialPort.DataBits = 8;
            mySerialPort.Handshake = Handshake.None;
            mySerialPort.ReadTimeout = 100;

            //mySerialPort.DataReceived += new SerialDataReceivedEventHandler(DataReceivedHandler);
            mySerialPort.Open();

            fifo_peekonly = new ConcurrentQueue<byte>();
            fifo_queue = new BlockingCollection<byte>(fifo_peekonly);

            mySerialPort.DataReceived += (sender, e) =>
            {
                byte[] buffer = new byte[mySerialPort.BytesToRead];
                if (!mySerialPort.IsOpen)
                {
                    throw new System.InvalidOperationException("Serial port is closed.");
                }
                mySerialPort.Read(buffer, 0, mySerialPort.BytesToRead);
                foreach (var b in buffer)
                    fifo_queue.Add(b);
            };

            Thread thr = new Thread(new ThreadStart(mythread));
            thr.Start();
        }

        public byte GetByteFromDevice()
        {
            byte b;
            b = fifo_queue.Take();
            return b;
        }

       public void mythread()
        {
            while(true)
            {
                if(recvByteCnt == 0)
                {
                    recvLength = GetByteFromDevice();
                    recvByte = recvLength;
                }
                else
                {
                    recvByte = GetByteFromDevice();
                }

                recvBuffer[recvByteCnt] = recvByte;

                if (recvByteCnt ≥ recvLength)
                {
                    recvFlag = 1;
                    recvByteCnt = 0;

                    Debug.Write("Complete\n");
                }
                else
                {
                    if (recvByteCnt == 0)
                    {
                        Debug.Write("Start timeout\n");
                    }
                    recvByteCnt++;
                }

                //Debug.Write(recvByte.ToString() + "\n");
            }
        }