function makeRequest(url) {
 http_request = false;
 if (window.XMLHttpRequest) {
  http_request = new XMLHttpRequest();
  if (http_request.overrideMimeType){
   http_request.overrideMimeType('text/xml');
  } 
 } else if (window.ActiveXObject) {
  try{
   http_request = new ActiveXObject("Msxml2.XMLHTTP"); 
  } catch (e) {
   try {
    http_request = new ActiveXObject("Microsoft.XMLHTTP");
   } catch (e) {
   }
  }
 } 
 if (!http_request) {
  alert("您的浏览器不支持当前操作，请使用 IE 5.0 以上版本!");
  return false;
 }
 

//定义页面调用的方法init,不是init();没有();
 http_request.onreadystatechange = init; 
 http_request.open('GET', url, true); 

//禁止IE缓存
 http_request.setRequestHeader("If-Modified-Since","0"); 

//发送数据
 http_request.send(null);

//每5秒刷新一次页面
 setTimeout("makeRequest('"+url+"')", 5000); 
}

function init() { 
 if (http_request.readyState == 4) {
  if (http_request.status == 0 || http_request.status == 200) {
   var result = http_request.responseText;
   if(result==""){
    result = "获取信息失败";
   }
   document.getElementById ("sysTimeShow").innerHTML=result; 
  } else {//http_request.status != 200
//   alert("请求失败!");
  }
 }
}