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>