// !!! Attention !!!
void initPort (SerialPort port) {
  // set this encoding to extend the ascii range
  port.Encoding = System.Text.Encoding.GetEncoding(28591);
}

// return default quat if packet is invalid
Quaternion getQuatFromString (string s, Quaternion defaultQuat) {
  char[] packet = s.ToCharArray ();
  string str = "";
  foreach (char c in packet) {
    str += (int)c + " ";
  }
    //Debug.Log (serialRead + str);

  // Windows user: use (packet.Length != 18 || packet[0] != 36) as condition
  if (packet.Length != 19 || packet[0] != 36 || packet[18] != 13) {
    return defaultQuat;
  } else {
    float[] q = new float[4];
    q[0] = ((packet[2] << 8) | packet[3]) / 16384f;
    q[1] = ((packet[4] << 8) | packet[5]) / 16384f;
    q[2] = ((packet[6] << 8) | packet[7]) / 16384f;
    q[3] = ((packet[8] << 8) | packet[9]) / 16384f;
    for (int i = 0; i < 4; i++) if (q[i] >= 2) q[i] = -4 + q[i];

    //Debug.Log (quat.x + " " + quat.y + " " + quat.z + " " + quat.w);
    return new Quaternion (q [1], q [2], q [3], q [0]);
  }
}

// sample usage
void readFromArduinoPort (SerialPort serial) {
  string serialRead = serial.ReadLine ();
  obj.rotation = getQuatFromString (serialRead, obj.rotation);
}