RXTX serial port read bytes

Question

I'm trying to send command and read from rs 232 with no flow control. Sending command is successful (checked by Serial port monitoring). but no answer. when I send the exact same command from Serial port monitoring software it answers. Dose any one have any idea why?

public static void main(String[] args) {
    SerialPort serialPort = new SerialPort("COM10");
    try {

      int mask = SerialPort.MASK_RXCHAR + SerialPort.MASK_CTS + SerialPort.MASK_DSR + SerialPort.MASK_RING;


      serialPort.openPort();//Open serial port
      serialPort.setParams(9600, 8, 1, 0);//Set params.
      serialPort.setEventsMask(mask);// Set mask
      serialPort.setFlowControlMode(SerialPort.FLOWCONTROL_XONXOFF_IN | SerialPort.FLOWCONTROL_XONXOFF_OUT );


      String command = "~ÿGjO~";          
        char[] chars = command.toCharArray();
        StringBuilder hex = new StringBuilder();
        for (char ch : chars) {
            hex.append(Integer.toHexString((int) ch));
        }

        System.out.println(command);
        serialPort.writeBytes(command.getBytes());

      byte[] buffer = serialPort.readBytes(16);//Read 10 bytes from serial port
      serialPort.closePort();//Close serial port
    }
    catch (SerialPortException ex) {
      System.out.println(ex);
    }
  }