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



namespace Serial_Port_COM
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
            getAvailablePorts();
        }
        void getAvailablePorts()
        {
            String[] ports = SerialPort.GetPortNames();
            comboBox1.Items.AddRange(ports);
        }
        private void ComputeCoefficients(int NumOfPulses,int[] coeffs)
        {
            coeffs[0] = NumOfPulses / 10000;
            coeffs[1] = (NumOfPulses % 10000) / 100;
            coeffs[2] = (NumOfPulses % 10000) % 100;
        }
        private void label1_Click(object sender, EventArgs e)
        {

        }

        private void Form1_Load(object sender, EventArgs e)
        {

        }

        private void groupBox1_Enter(object sender, EventArgs e)
        {

        }

        private void buttonOpen_Click(object sender, EventArgs e)
        {
            try
            {
                if (comboBox1.Text == "" || comboBox2.Text == "")
                {
                    textBox2.Text = " please slect port setting";
                }
                else
                {
                    serialPort1.PortName = comboBox1.Text;
                    serialPort1.BaudRate = Convert.ToInt32(comboBox2.Text);
                    serialPort1.Open();
                    progressBar1.Value = 100;
                    buttonSent.Enabled = true;
                    buttonRead.Enabled = true;
                    textBox1.Enabled = true;
                    buttonOpen.Enabled = false;
                    buttonClose.Enabled = true;
                }
            }
            catch (UnauthorizedAccessException)
            {
                textBox2.Text = "unauthorized Access";
            }
        }

        private void buttonClose_Click(object sender, EventArgs e)
        {
            serialPort1.Close();
            progressBar1.Value = 0;
            buttonSent.Enabled = false;
            buttonRead.Enabled = false;
            buttonOpen.Enabled = true;
            buttonClose.Enabled = false;
            textBox1.Enabled = false;
        }
        private void SendDataToSerialPort(int[] paramaters,int length)
        {
            if (this.serialPort1.IsOpen)
            {
                serialPort1.DiscardInBuffer();
                serialPort1.DiscardOutBuffer();
                byte[] buf = new byte[length];
                for (int i = 0; i < length; i++)
                {
                    byte[] temp = BitConverter.GetBytes(paramaters[i]);
                    buf[i] = temp[0];
                }

                serialPort1.Write(buf, 0, length);
                //buf[0] =label4.Text;
            }
        }
        private void buttonSent_Click(object sender, EventArgs e)
        {
            //int NumOfPulses = int.Parse(this.textBox1.Text);
            int NumOfPulses = Convert.ToInt32(textBox1.Text);
           // int NumOfPulses = 0;
           // if(Int32.TryParse(textBox1.Text, out NumOfPulses))
                //NumOfPulses          
            int[] coeffs = { 0, 0, 0 };
            ComputeCoefficients((int)NumOfPulses, coeffs);
            int[] paramaters = new int[3];
            paramaters[0] = coeffs[0];
            paramaters[1] = coeffs[1];
            paramaters[2] = coeffs[2];
            int length = 3;
            SendDataToSerialPort(paramaters,length);
            textBox1.Text = "da gui";
        }

        private void buttonRead_Click(object sender, EventArgs e)
        {
            try
            {
                textBox2.Text = serialPort1.ReadLine();
            }
            catch(TimeoutException)
            {
                textBox2.Text = "khong nhan dc";
            }
        }
    }
}
.
code aruino:
void setup() 
{
  
  Serial.begin(9600);
  pinMode(13,OUTPUT);
  digitalWrite(13,LOW);// put your setup code here, to run once:

}

void loop() 
{
  char mybytes[3];
  for(int i=0;i<3;i++)
  {
  mybytes[i]=Serial.read();
  // put your main code here, to run repeatedly:
  }
  int a;
  a=mybytes[2]+mybytes[1]*100+mybytes[0]*10000;
  if(a==9600)
  {
    digitalWrite(13,HIGH);
  }
  if(a==0)
  {
    digitalWrite(13,LOW);
  }
}  
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