Life is an algorithm which is recursive...

    <script type="text/javascript" language="JavaScript">

    function textCounter(field,cntfield,maxlimit) {

        if (field.value.length > maxlimit)

            field.value = field.value.substring(0, maxlimit);

        else

            cntfield.value = maxlimit - field.value.length;

       }

    </script>

       <table>

           <tr>

               <td>

                   <input readonly type="text" name="Sayac" maxlength="3" value="230" style="width: 29px" />

                   Kalan karakter sayısı

               </td>

           </tr>

           <tr>

               <td>

                   <asp:TextBox runat="server" ID="txtKisitli" Width="250px" Height="100px" TextMode="MultiLine"

                       onKeyDown="textCounter(document.aspnetForm.txtKisitli,document.aspnetForm.Sayac,230)"

                       onKeyUp="textCounter(document.aspnetForm.txtKisitli,document.aspnetForm.Sayac,230)">

                   </asp:TextBox>

               </td>

           </tr>