How to get Characters -> Get Bytes

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.IO.Ports;
using System.Threading;

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 = "COM3";
            serialPort1.BaudRate = 9600;
            serialPort1.Parity = Parity.None;
            serialPort1.DataBits = 8;
            serialPort1.StopBits = StopBits.One;
            serialPort1.Handshake = Handshake.None;
                
            serialPort1.Open();
            if (serialPort1.IsOpen)
            {
                serialPort1.WriteTimeout = 500;
                buttonStart.Enabled = false;
                buttonStop.Enabled = true;
                textBox1.ReadOnly = false;
            }
        }
        const char STX = '\u0002';
        const char ETX = '\u0003';
        readonly string pull_shelf_104 = string.Format("{0}01P00104##{1}" , STX, ETX);

        private byte[] WrapString(string pull_shelf_104)
        {
            int length = pull_shelf_104.Length;
            byte[] send104 = new byte[length];
            Array.Copy(System.Text.Encoding.ASCII.GetBytes(pull_shelf_104), 0, send104, 0, length);
            return send104;
        }
        
        private void linkLabel_HC1_100_LinkClicked(object sender, LinkLabelLinkClickedEventArgs e)
        {
            if (serialPort1.IsOpen)
            {
                byte[] data = WrapString(pull_shelf_104);
                serialPort1.Write(data,0,12);
            }
        }
        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 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));
        }
    }
}