Not able to read COM3 port. Always output is 0

Question

This code is able to run, however it only return 0 even though the device has some value. The device is a roller machine where it should give the rolling meter value to the port number 3 however, my code always return the output as 0.
Please help me as soon as possible as this is a critical requirement.
using System;
using System.Text;
using System.IO.Ports;
using System.Collections.Generic;
namespace ConsoleApplication3
{
    class Program
    {
        static SerialPort sp;
        string InputData = string.Empty;
        static void Main(string[] args)
        {
            sp = new SerialPort("COM3", 4800, Parity.None, 8, StopBits.One);
            sp.DiscardNull = false;
            sp.RtsEnable = true;
            sp.ReadTimeout = 500;
            sp.Handshake = Handshake.None;
            sp.Encoding = Encoding.UTF8;
            sp.DtrEnable = true;
            sp.DataReceived += new SerialDataReceivedEventHandler(sp_DataReceived);
            sp.Open();
            //sp.DataReceived += sp_DataReceived;
            string InputData = sp.ReadExisting();
            while (sp.IsOpen)
            {
                try
                {
                    System.Threading.Thread.Sleep(300);
                    int bytes = sp.BytesToRead;
                    byte[] buffer = new byte[bytes];
                    Console.WriteLine("Value - " + sp.Read(buffer, 0, bytes));
                }
                catch (Exception ex)
                {
                    Console.WriteLine(ex.Message);
                }
            }
        }
        private static void sp_DataReceived(object sender, SerialDataReceivedEventArgs e)
        {
            SerialPort sport = (SerialPort)sender;
            string indata = sport.ReadExisting();
            Console.WriteLine(indata);
        }
    }
}