port.Write(a, 0, 1);

port.Write(textBox1.Text,0,1);

code 
data 
rx 
sfr 
ldm

System.Text.ASCIIEncoding  encoding=new System.Text.ASCIIEncoding();
    Byte[] bytes = encoding.GetBytes(textBox1.Text);

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;

namespace ser33
{
    public partial class Form1 : Form

    {
        SerialPort port = new SerialPort("COM1", 9600, Parity.None, 8, StopBits.One);
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            string x;
            x = "a";
            /*int anInteger;
            anInteger = Convert.ToInt32(textBox1.Text);
            anInteger = int.Parse(textBox1.Text);
            char[] ar1 ={ textBox1.Text };*/
            System.Text.ASCIIEncoding encoding = new System.Text.ASCIIEncoding();
            Byte[] bytes = encoding.GetBytes(textBox1.Text);


          
            port.Open();
            port.Write(x);
            port.Write(bytes,0,1);
            port.Close();
        }

        private void button2_Click(object sender, EventArgs e)
        {
            string z;
            z = "b";

           /* int anInteger2;
            anInteger2 = Convert.ToInt32(textBox2.Text);
            anInteger2 = int.Parse(textBox2.Text);
            char[] ar2 ={ textBox2.Text };*/
            System.Text.ASCIIEncoding encoding = new System.Text.ASCIIEncoding();
            Byte[] bytes = encoding.GetBytes(textBox2.Text);

            port.Open();
            port.Write(z);
            port.Write(bytes,0,1);
            
            port.Close();
        }

        private void button3_Click(object sender, EventArgs e)
        {
            this.Close();
        }
    }
}

data p1,p2,p11,p22;

void main()
{
 int i,y;
trisb=0;
portb=0;
uart1_init(9600);
delay_ms(500);

while (1)
{ 
if(uart1_data_ready())
{
                   p1=uart1_read();
                   if (p1 == 'a') {
                   p11=uart1_read();
  for(i=1;i<=p11;i++)
{            portb.b3=1;
   vdelay_ms(1000);
           }
            portb.b3=0;
           }
           if (p1 =='b'){
           p22=uart1_read();
  for(y=1;y<=p22;y++)
{
   portb.b2=1;
   vdelay_ms(1000);
          }
            portb.b2=0;
           
          }}

}}
و لك جزيل الشكر

int p2,p11,p22;
char p1;

void main()
{
int i,y;
trisb=0;
portb=0;
uart1_init(9600);
delay_ms(500);

while (1)
{
if(uart1_data_ready())
{
                   p1=uart1_read();
                   if (p1 == 'a') {
                   while(uart1_data_ready()!=1){}
                   p11=uart1_read();
  for(i=1;i<=p11;i++)
{            portb.b3=1;
  delay_ms(1000);
           }
            portb.b3=0;
           }
           if (p1 =='b'){
           while(uart1_data_ready()!=1){}
           p22=uart1_read();
  for(y=1;y<=p22;y++)
{
   portb.b2=1;
   delay_ms(1000);
          }
            portb.b2=0;

          }
          }

}
}

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;

namespace ser33
{
    public partial class Form1 : Form

    {
        Byte[] bytes = new byte[1] { 0 };
        SerialPort port = new SerialPort("COM1", 9600, Parity.None, 8, StopBits.One);
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            string x;
            x = "a";
            /*int anInteger;
            anInteger = Convert.ToInt32(textBox1.Text);
            anInteger = int.Parse(textBox1.Text);
            char[] ar1 ={ textBox1.Text };*/
            //System.Text.ASCIIEncoding encoding = new System.Text.ASCIIEncoding();
            //Byte[] bytes = encoding.GetBytes(textBox1.Text);
            bytes[0] = byte.Parse(textBox1.Text);
            port.Open();
            port.Write(x);
            System.Threading.Thread.Sleep(100);
            port.Write(bytes,0,1);
            port.Close();
        }

        private void button2_Click(object sender, EventArgs e)
        {
            string z;
            z = "b";

           /* int anInteger2;
            anInteger2 = Convert.ToInt32(textBox2.Text);
            anInteger2 = int.Parse(textBox2.Text);
            char[] ar2 ={ textBox2.Text };*/
           // System.Text.ASCIIEncoding encoding = new System.Text.ASCIIEncoding();
            //Byte[] bytes = encoding.GetBytes(textBox2.Text);
            bytes[0] = byte.Parse(textBox2.Text);
            port.Open();
            port.Write(z);
            System.Threading.Thread.Sleep(100);
            port.Write(bytes,0,1);
            
            port.Close();
        }

        private void button3_Click(object sender, EventArgs e)
        {
            this.Close();
        }
    }
}

data da,da1;
void main(){
trisb=0;
trisd=0;
portd=0;
portb=0;
portb.b1=1;portb.b5=1;
delay_ms(1000);
portb.b1=0;portb.b5=0;
uart1_init(9600);
delay_ms(100);
while(1)
{
if(uart1_data_ready())
{
da=uart1_read();
if (da='r')

{
portb.b1=1;
portb.b5=1;
portd.b0=1;
}
else if (uart1_data_ready())
{
da=  uart1_read();
if (da='s')


{
portd.B2=1;
portb.B2=1;
    }}                    }


   }
}

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;

namespace serialp22
{
    public partial class Form1 : Form
    {



       Byte[] bytes = new byte[1] { 0 };


        SerialPort port = new SerialPort("COM1", 9600, Parity.None, 8, StopBits.One);
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
           //string n1;
            //n1 = "D";
            //textBox1.Text = "1";

           //int x1 = 1;
          // bytes[0] = byte.Parse(textBox1.Text);

          
           //bytes = BitConverter.GetBytes(x1);


            port.Open();
        port.Write("r");
         System.Threading.Thread.Sleep(100);

            //port.Write(bytes, 0, 1);
            port.Close();
        }

        private void button2_Click(object sender, EventArgs e)
        {
          //  textBox2.Text = "0";
            //string n2;
            //n2 = "Y";
          // int x2 = 0;
           // bytes[0] = byte.Parse(textBox1.Text);
            //bytes = BitConverter.GetBytes(x2);
            //bytes[0] = byte.Parse(textBox2.Text);
            if (port.IsOpen)
                port.Close();
            port.Open();
            port.Write("s");
         System.Threading.Thread.Sleep(100);

           //port.Write(bytes, 0, 1);
         port.Close();
        }

        
    }
}