Send xmlHttpRequest every 10 second in JavaScript

I run a JavaScript function that send a xmlHttpRequest to an .ashx (let's name it send_req() that run on page load for first time). For onreadystatechange, I have a function that receive XML data and show it on the page (let's name this one getanswer()).

I want to automatically update XML data on the page every 20 seconds. For that, I use setTimeout(send_req(),20000) in the end of writexml(), but it doesn't update data on the page. I add an alert() at the **** line in the code. It shows on the page every one second!

And my code works fine if I use it without setTimeout.

Here is my code

var Population = "";
var Available_money = "";
var resource_timer;
var httpReq_resource;

function send_req() {
    if (windo开发者_开发百科w.ActiveXObject) {
        httpReq_resource = new ActiveXObject("Microsoft.XMLHTTP");
    }
    else if (window.XMLHttpRequest) {
        httpReq_resource = new XMLHttpRequest();
    }

    var sendStr = "user_id=1";
    if (httpReq_resource)
    {
        httpReq_resource.onreadystatechange = getanswer;
        httpReq_resource.open("POST", "Answer_Resource_change.ashx");
        httpReq_resource.send(sendStr);
    }
}

function getanswer() {
    var results = httpReq_resource.responseXML;
    if (httpReq_resource.readyState == 4) {
        if (httpReq_resource.status == 200) {
            try {
                var value;
                var values = results.getElementsByTagName("values");
                for (var i = 0; i < values.length; i++) {

                    value = values[i];
                    Population = value.getElementsByTagName("Population")[0].firstChild.nodeValue;
                    Available_money = value.getElementsByTagName("Available_money")[0].firstChild.nodeValue;
                    ... and some more like two line up
                }

                make_changes();
                **********************************
                resource_timer = setTimeout(send_req(), 20000);

            }
            catch (e) {
            }
        }
    }
}

function make_changes() {
    $("li span#l1").text(Available_money + '/' + Population);
    ...and some more like up line
}

This:

resource_timer = setTimeout(send_req(), 20000);

Should be:

resource_timer = setTimeout(send_req, 20000);

The first executes the result of send_req() after 20 seconds, the second executes send_req itself.