function abrirsistema(url){
  var pagina = url;
  var ancho=750; var alto=400;
  var posX= (screen.width-ancho)/2;
  var posY= (screen.height-(alto+70))/2;
  props= "toolbar=1,location=0,directories=0,status=0,menubar=0,scrollbars=yes,resizable=1,copyhistory=0,width="+ancho+",height="+alto+",left=" +posX+ ",top=" +posY;
  nuevaventana = window.open(url,'SISTEMAS',props);
}