como saber que o Servidor onde o XML esta , nao manda mais dados

Como Faço para saber quando nao esta mais chegando os DADOS XML ?

Codigo

<script type="text/javascript" language="javascript">var http_request = false;var i=0;function makeRequest(url) {http_request = false;if (window.XMLHttpRequest) { // Mozilla, Safari,... http_request = new XMLHttpRequest(); if (http_request.overrideMimeType) { http_request.overrideMimeType('text/xml'); // See note below about this line }} else if (window.ActiveXObject) { // IE try { http_request = new ActiveXObject("Msxml2.XMLHTTP"); } catch (e) { try {http_request = new ActiveXObject("Microsoft.XMLHTTP"); } catch (e) {} }}if (!http_request) { alert('Giving up :( Cannot create an XMLHTTP instance'); return false;}http_request.onreadystatechange = alertContents;http_request.ZXXXX = trataerros;http_request.open('GET', url, true);http_request.send(null);}function puxa_xml() { makeRequest('get.php');}var cnt = 1;function trataerros() { alert('opa');}function alertContents() { document.getElementById("evento").innerHTML=(http_request.readyState) + ' - ' + http_request.status; if (http_request.readyState == 4) { if (http_request.status == 200) {	// alert(http_request.responseText);	var xmldoc = http_request.responseXML;	var root_node = xmldoc.getElementsByTagName('linha').item(i); // alert(root_node.firstChild.data);	document.getElementById('teste').innerHTML = cnt + ' ' + (root_node.firstChild.data); // document.getElementById('teste_2').innerHTML = (root_node.firstChild.data);	cnt = cnt + 1;	puxa_xml();	i++;	if (i==3){ i=0;	} } else {	alert('There was a problem with the request.'); } } }</script><spanstyle="cursor: pointer; text-decoration: underline"onclick="puxa_xml()">Make a request</span><table border="1"><tr><td><div id='teste'>CallBOX</div></td><td><div id='evento'>CallBOX 2</div></td></tr></table>