serial port two c# applications not sending/receiving

private void Upload_Click(object sender, EventArgs e)
        {

           // StringBuilder sb = new StringBuilder();
  

            Excel.Application xlApp = new Excel.Application();
            Excel.Workbook xlWorkbook = xlApp.Workbooks.Open(textBox1.Text, 0, true, 5, "", "", true, Excel.XlPlatform.xlWindows, "\t", false, false, 0, true, 1, 0);
            Excel._Worksheet xlWorksheet = (Excel._Worksheet)xlWorkbook.Sheets[1];
            Excel.Range xlRange = xlWorksheet.UsedRange;

            int rowCount = xlRange.Rows.Count;
            int colCount = xlRange.Columns.Count;
            progressBar1.Visible = true;
            

            int k = 100 / rowCount;

            for (int i = 2; i <= rowCount; i++)
            {

               
                for (int j = 1; j <= colCount; j++)
                {

                    if (j == 1)
                     machId = xlRange.Cells[i, j].Value.ToString();
                 

                        //upload1 = xlRange.Cells[i, j].Value.ToString();
                    if (j == 2)
                        custId = xlRange.Cells[i, j].Value.ToString();
                   
                  
                    if (j == 3)
                        name = xlRange.Cells[i, j].Value.ToString();
                
                    if (j == 4)
                        totalBal= xlRange.Cells[i, j].Value.ToString();
                    
                     if (j == 5)
                        paid = xlRange.Cells[i, j].Value.ToString();
                     
                    if (j == 6)
                        lastPaid= xlRange.Cells[i, j].Value.ToString();
                    
                    if (j == 7)
                       due = xlRange.Cells[i, j].Value.ToString();


                    final = "$ POST," + machId + "," + custId + "," + name + "," + totalBal + "," + paid + "," + lastPaid + "," + due + " # ";


                }

                if (serialPort1.IsOpen)
                {
                    if(machId.Length<=5 && custId.Length<=10 && name.Length<=15 && totalBal.Length<=7 && paid.Length<=7 && lastPaid.Length<=14 && due.Length<=7 )
                  serialPort1.Write(final);
                    

            }     serialPort1.DataReceived += new SerialDataReceivedEventHandler(serialPort1_DataReceived);

<pre lang="c#">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 Hand_Held_Data_Transporter;
using System.IO.Ports;
using System.Threading;

namespace handheld_Responce
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
            serialPort1.Open();
            serialPort1.DataReceived += new System.IO.Ports.SerialDataReceivedEventHandler(serialPort1_DataReceived);
     
       
        }

        private void send_Click(object sender, EventArgs e)
        {

        }

        private void Form1_Load(object sender, EventArgs e)
        {




        }

        private void serialPort1_DataReceived(object sender, SerialDataReceivedEventArgs e)
        {
           
                if (serialPort1.IsOpen)
                {
                    String RecievedData;
                    RecievedData = serialPort1.ReadExisting();
                    if (!(RecievedData == ""))
                    {
                        //textBox2.Text += RecievedData;
                        textBox2.Invoke((MethodInvoker)delegate { textBox2.Text += RecievedData; });
                    }
                    Thread.Sleep(100);
                    serialPort1.WriteLine("$M121,0#");
            }
            
        }
    }
}