Maybe something along the lines of this (untested)
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<meta http-equiv"Script-Content-Type" content="text/javascript">
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<meta http-equiv="Expires" content="0"> <!-- disable caching -->
<title>Example</title>
<script type="text/javascript">
timerHandle = null;
value = null;
function showPress(val) {
// Set the global variable 'value' to the value which the user
// selected. It has been kept global so that it can be used
// by the timeOut function even when there is no user event
// and the displayTable is called on the latest
// value from drop down.
value = val;
// Firstly populate the contents of the table.
displayTable();
// Create a new timer only if one doesn't already exist which
// would call the displayTable function at an interval of
// 30 seconds starting now.
if(!timerHandle) {
timerHandle = setInterval(displayTable, 30000);
}
}
function displayTable() {
// Make an ajax request using 'value' and check the status
document.getElementById("txtHint").innerHTML = xmlHttp.responseText
}
</script>
</head>
<body>
<form id="frm" name="frm" action="#">
<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>
</body>
</html>