numpos = 4;
function fAleatorio(a,b) {
	return Math.round(Math.random()*(b-a)+a);
}//Fin function fAleatorio

function fMuestra(num){
	for (var k = 0; k < numpos; k++) document.getElementById('vol' + (k+1)).style.display='none';
	document.getElementById( 'vol' + num ).style.display='block';
}/*Fin fMuestra*/

function fFirst(){
	num = fAleatorio(1,numpos);
	fMuestra(num);
	fRotar(num);
}//Fin function fFirst

function fRotar(){
	num++;
	if (num > numpos) num = 1;
	fMuestra(num);
} //Fin FUNCTION fRotar...
	timer = setInterval("fRotar()", 8000);