var y = 1;
var step = 5;
var down = true;
var minstep=step;
var running=false;
var t

function smooth_on_off(p_id, hmax){
running=true;
obj=document.getElementById(p_id);

if(y>hmax&&down){step=-step;
down=false;
running=false;
clearInterval(t);
return false;}
if(y<minstep&&!down){step=-step;
down=true;
running=false;
y=1;
clearInterval(t);
return false;}

y = y+step;

obj.style.height = y + 'px';
}

function deroule(p_id,hmax){
if (running) {return false;}
t=setInterval(function(){smooth_on_off(p_id,hmax)},1)

}