ช่วยดูให้หน่อยครับ เขียนโปรแกรมตามตัวอย่าง ส่งค่าได้แต่กลับรับค่ามาแสดงผลไม่ได้ครับ RS232

 using System;
  using System.Collections.Generic;
  using System.ComponentModel;
  using System.Data;
  using System.Drawing;
  using System.Text;
  using System.Windows.Forms;
  
  namespace SimpleSerial
  {
      public partial class Form1 : Form
      {
          // Add this variable

          string RxString;
          
          public Form1()
          {
              InitializeComponent();
          }
  
          private void buttonStart_Click(object sender, EventArgs e)
          {
              serialPort1.PortName = "COM1";
              serialPort1.BaudRate = 9600;
  
              serialPort1.Open();
              if (serialPort1.IsOpen)
              {
                  buttonStart.Enabled = false;
                  buttonStop.Enabled = true;
                  textBox1.ReadOnly = false;
              }
          }
  
          private void buttonStop_Click(object sender, EventArgs e)
          {
              if (serialPort1.IsOpen)
              {
                  serialPort1.Close();
                  buttonStart.Enabled = true;
                  buttonStop.Enabled = false;
                  textBox1.ReadOnly = true;
              }
  
          }
  
          private void Form1_FormClosing(object sender, FormClosingEventArgs e)
          {
              if (serialPort1.IsOpen) serialPort1.Close();
          }
  
          private void textBox1_KeyPress(object sender, KeyPressEventArgs e)
          {
              // If the port is closed, don't try to send a character.

              if(!serialPort1.IsOpen) return;
  
              // If the port is Open, declare a char[] array with one element.
              char[] buff = new char[1];
              
              // Load element 0 with the key character.

              buff[0] = e.KeyChar;
  
              // Send the one character buffer.
              serialPort1.Write(buff, 0, 1);
  
              // Set the KeyPress event as handled so the character won't
              // display locally. If you want it to display, omit the next line.
              e.Handled = true;
          }
  
          private void DisplayText(object sender, EventArgs e)
          {
              textBox1.AppendText(RxString);
          }
  
          private void serialPort1_DataReceived
            (object sender, System.IO.Ports.SerialDataReceivedEventArgs e)
          {
              RxString = serialPort1.ReadExisting();
              this.Invoke(new EventHandler(DisplayText));
          }
      }
  }
  

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 WindowsFormsApplication3
{
    public partial class Form1 : Form
    {
        string RxString;
        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            serialPort1.PortName = "COM1";
            serialPort1.BaudRate = 9600;
            serialPort1.DataBits = 8;
            serialPort1.Parity = Parity.None;
            serialPort1.StopBits = StopBits.One;                
          }

        private void button1_Click(object sender, EventArgs e)
        {
            serialPort1.Open();
            serialPort1.Write("start");
            serialPort1.Close();

        }      

        private void DisplayText(object sender, EventArgs e)
        {
            textBox1.AppendText(RxString);
        }

        private void check(object sender, System.IO.Ports.SerialDataReceivedEventArgs e)
        {
            RxString = serialPort1.ReadExisting();
            this.Invoke(new EventHandler(DisplayText));
        }

       

    }
}