ในโปรแกรมเป็นรับค่าเป็น Char   แต่ผมจะทำการรับค่าเป็นตัวเลขปกติ
ไม่ทราบว่าจะต้องแก้ตรงส่วนไหนบ้างครับ
ขอบคุณล่วงหน้าครับ

using System;
using System.Windows.Forms;

namespace RS232
{
   public partial class Form2 : Form
   {
       private System.IO.Ports.SerialPort serialPort1;
       //private IContainer components;
       private TextBox textBox1;
       private Button buttonStart;
       private Button buttonStop;
       // Add this variable
       string RxString;

       public Form2()
       {
           
           InitializeComponent();
       }

       private void buttonStart_Click(object sender, EventArgs e)
       {
           serialPort1.PortName = "COM13";
           serialPort1.BaudRate = 9600;
           serialPort1.DataBits = 8;
           serialPort1.StopBits = System.IO.Ports.StopBits.One;
           serialPort1.Parity = System.IO.Ports.Parity.None;
           serialPort1.Handshake = System.IO.Ports.Handshake.RequestToSend;
           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));
       }



   }
}