function makeHttpRequest(url, callback_function) {
    var http_request=false;

    if(window.XMLHttpRequest) { // Mozilla, Safari, ...
        http_request=new XMLHttpRequest();
    } else if(window.ActiveXObject) { // Internet Explorer
        try {
            http_request=new ActiveXObject("Msxml2.XMLHTTP");
        } catch (e) {
            try {
                http_request=new ActiveXObject("Microsoft.XMLHTTP");
            } catch (e) {}
        }
    }

    if(!http_request) { return false; }

    http_request.onreadystatechange=function() {
        if (http_request.readyState==4) {
            if (http_request.status==200) {
                eval(callback_function+'(http_request.responseText)');
            }
        }
    }
    http_request.open('GET', url, true);
    http_request.send(null);
}

function loadBanner(response) {
    //Stampa immagine risultato
    document.getElementById('risultato').innerHTML=response;

    try {
        clearTimeout(to);
    } catch (e) {}

    //Tempo transizione in ms
    to=setTimeout("nextAd()", 6000);
}

function nextAd() {
    var now=new Date();
    var url='banner.php?ts='+now.getTime();
    makeHttpRequest(url, 'loadBanner');
}

window.onload=nextAd;