Serial Data Communication using Unity

Can anyone here guide me step by step how to read data form serial port i.e Serial Data Communication with Unity? Any help would be much appreciated. Thank you. I get it worked but why my code shows only 255 as output when data received? My code is:

 using UnityEngine;
 
 using System.Collections;
 
 #region Using directives
 
 using System;
 
 using System.Collections.Generic;
 
 using System.ComponentModel;
 
 using System.IO.Ports;
 
 #endregion
 
  
 public class SerialPort : MonoBehaviour {
 
    private SerialPort sp;
 
    void Start () {
 
       sp = new SerialPort( "COM3", 9600, Parity.None, 8, StopBits.One);
 
       sp.Open();
 
       sp.ReadTimeout = 50;
 
    }
 
  
 
    void OnApplicationQuit() {
 
       sp.Close();
 
    }
 
    void FixedUpdate() {
 
             string tempS = "";
 
        try {
 
          byte tempB =  (byte) sp.ReadByte();
 
            while (tempB != 255) {
 
                tempS += ((char) tempB);
 
                tempB = (byte) sp.ReadByte();
 
            }
 
        } 
 
        catch (Exception e) {
 
        }
 
         if (tempS!="") {
 
               Debug.Log("serial out "+tempS);
 
         }    
    }
 
 }