A little help modifying some AJAX - need setTimeout() Guidance

[jugo]

I have this code that automatically displays a table with mysql data depending on the selection from a form.

I need to add code to make the table refresh every 30 seconds using the parameter selected in the form. I have googled this and after trying every single way I know to implement the setTimeout() function, I have almost given up.

Can anyone please help me here....

included_script.js


var xmlHttp

function showPress(str)
{
xmlHttp=GetXmlHttpObject()
if (xmlHttp==null)
{
alert ("Browser does not support HTTP Request")
return
}
var url="_data.php"
url=url+"?q="+str
url=url+"&sid="+Math.random()
xmlHttp.onreadystatechange=stateChanged
xmlHttp.open("GET",url,true)
xmlHttp.send(null)
}

function stateChanged()
{
if (xmlHttp.readyState==4 || xmlHttp.readyState=="complete")
{
document.getElementById("txtHint").innerHTML=xmlHttp.responseText

}
}


function GetXmlHttpObject()
{
var xmlHttp=null;
try
{
// Firefox, Opera 8.0+, Safari
xmlHttp=new XMLHttpRequest();
}
catch (e)
{
//Internet Explorer
try
{
xmlHttp=new ActiveXObject("Msxml2.XMLHTTP");
}
catch (e)
{
xmlHttp=new ActiveXObject("Microsoft.XMLHTTP");
}
}
return xmlHttp;
}


HTML form


<form>
Select:
<select name="eqno" onchange="showPress(this.value)">
<option value="1" >Number 1</option>
<option value="2" >Number 2</option>
<option value="3" >Number 3</option>
</select>
</form>