class Program
 {
  static SerialPort port;
  static Byte[] SerialDataIn = null;

  static void Main(string[] args)
  {
   port = new SerialPort("COM7", 250000, Parity.None, 8, StopBits.One);
   port.Handshake = Handshake.None;
   port.DataReceived += new SerialDataReceivedEventHandler(port_DataReceived);
   port.Open();

   byte[] a = new byte[4] { 50, 51, 52, 53 };

   port.Write(a, 0, 4);

   while (true)
   {
    if (SerialDataIn != null)
    {
     for (int i = 0; i < 4; i++)
      Console.WriteLine("{0}: {1}", i, SerialDataIn[i]);
     SerialDataIn = null;
    }
    System.Threading.Thread.Sleep(100);
   }

   Console.WriteLine("ok");

   port.Close();
  }

  static void port_DataReceived(object sender, SerialDataReceivedEventArgs e)
  {
   if (port.BytesToRead >= 4)
   {
    SerialDataIn = new byte[4];
    port.Read(SerialDataIn, 0, 4);
   }
  }
 }