C# + COM คือจะเขียนยังไงครับให้เอาค่าที่รับมาแล้วส่งกลับไปครับ rs232

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.IO.Ports;

namespace WindowsFormsApplication9
{
    public partial class Form1 : Form
    {
        string RxString;
        private SerialPort COM_port = new SerialPort();
        public Form1()
        {
            InitializeComponent();
            COM_port.DataReceived += new SerialDataReceivedEventHandler(COM_port_DataReceived);

        }

     
        private void Form1_Load(object sender, EventArgs e)
        {

            COM_port.PortName = "COM1";
            COM_port.BaudRate = 9600;
            COM_port.Parity = (Parity)Enum.Parse(typeof(Parity), "None");
            COM_port.StopBits = (StopBits)Enum.Parse(typeof(StopBits), "One");
            COM_port.DataBits = int.Parse("8");
            COM_port.Open();           
         
        }
        void COM_port_DataReceived(object sender, SerialDataReceivedEventArgs e)
        {
            this.Invoke(new EventHandler(delegate
            {
                           
                               
                               RxString = COM_port.ReadExisting();
                            
                               COM_port.Write(RxString);    
            }
            )
            )
            ;
        }

        private void Send_button_Click(object sender, EventArgs e)
        {
            if (SendText.Text != "")
                COM_port.Write(SendText.Text);       
        }

    }
}