function set_ajax()
{
    var regresa_p;
    if (window.XMLHttpRequest) 
        regresa_p = new XMLHttpRequest();
    else
        if (window.ActiveXObject){ 		
            try{ regresa_p = new ActiveXObject("Msxml2.XMLHTTP"); } catch (e){ try{ regresa_p = new ActiveXObject("Microsoft.XMLHTTP");} catch (e){} }
        }
    else
        return false;
    return regresa_p;
}
function get_form_values (form)
{
	var str = "";
	var forma = document.getElementById( form );
	for(var i = 0;i < forma.elements.length; i++)
		{
		if( !forma.elements[i].disabled && forma.elements[i].type != "button" )
			 {
			if (forma.elements[i].type == "checkbox")
				{
				if (forma.elements[i].checked)
					{
					str += forma.elements[i].name + '=' + forma.elements[i].value + '&';
					}
				}
			else if (forma.elements[i].type == "select-multiple")
				{
				for(j=0; j < forma.elements[i].length; j++)
					{
					if (forma.elements[i].options[j].selected)
						{
						str += forma.elements[i].name + '[]=' + forma.elements[i].options[j].value + '&';
						}
					}
				}
			else
				{
				str += forma.elements[i].name + '=' + forma.elements[i].value + '&';
				}
			}
		}
	str = str.substr(0,(str.length - 1));
	return str;
	
}
function llamarasincrono(url, id_contenedor, params)
{
    var id_cont=document.getElementById(id_contenedor);
    if (id_cont)
    {
        var pagina_requerida = false;
        pagina_requerida=set_ajax();
        pagina_requerida.onreadystatechange=function(){
            if (pagina_requerida.readyState==1)
            {                
                id_cont.innerHTML = "<img src='/principal/llamadas/load.gif' align='center'> ";                
            }
            if (pagina_requerida.readyState==4){
                if(pagina_requerida.status==200)
                {
                    id_cont.innerHTML = pagina_requerida.responseText;	
                }
                else
                    if(pagina_requerida.status==404)
                        id_cont.innerHTML = "La direccion "+url+" no existe";
                else
                    id_cont.innerHTML = "Error: ".pagina_requerida.status;
            }
        }
        valorpost="pet=on&"+params;
        pagina_requerida.open('POST', url, true);
        pagina_requerida.setRequestHeader('Content-Type','application/x-www-form-urlencoded; charset=iso-8859-1');
        pagina_requerida.send(valorpost);
    }       
}