AJAX refres viac častí stránky v iných časových intervaloch

<script type="text/javascript">
var xmlhttp = false ;


if (!xmlhttp && typeof XMLHttpRequest != 'undefined')
{
try {
xmlhttp = new XMLHttpRequest ();
}
catch (e) {
xmlhttp = false}
}


function myXMLHttpRequest ()
{
var xmlhttplocal;
try {
xmlhttplocal = new ActiveXObject ("Msxml2.XMLHTTP")}
catch (e) {
try {
xmlhttplocal = new ActiveXObject ("Microsoft.XMLHTTP")}
catch (E) {
xmlhttplocal = false;
}
}

if (!xmlhttplocal && typeof XMLHttpRequest != 'undefined') {
try {
var xmlhttplocal = new XMLHttpRequest ();
}
catch (e) {
var xmlhttplocal = false;
}
}
return (xmlhttplocal);
}


var mnmxmlhttp = Array ();
var mnmString = Array ();
var mnmPrevColor = Array ();
var responsestring = Array ();
var myxmlhttp = Array ();
var responseString = new String;




var i=0;
var ii = 0;

function ajax_update()
{
url = "refresh_page1.php";
target2 = document.getElementById ('main_windows');
ii = i++;
var main_windows = "i=" + ii ;
mnmxmlhttp = new myXMLHttpRequest ();
if (mnmxmlhttp) {
mnmxmlhttp.open ("POST", url, true);
mnmxmlhttp.setRequestHeader ('Content-Type',
'application/x-www-form-urlencoded');
mnmxmlhttp.send (main_windows);
errormatch = new RegExp ("^ERROR:");
target2 = document.getElementById ('main_windows');
mnmxmlhttp.onreadystatechange = function () {
if (mnmxmlhttp.readyState == 4) {
mnmString = mnmxmlhttp.responseText;
if (mnmString.match (errormatch)) {
mnmString = mnmString.substring (6, mnmString.length);
target = document.getElementById ('main_windows');
target2.innerHTML = mnmString;
} else {
target = document.getElementById ('main_windows');
target2.innerHTML = mnmString;
}
}
}
}
setTimeout('ajax_update()', 1000);
}
</script>