var onTopScrollPointer, onDownScrollPointer;
var myspeed = 0, speed = 2;

function getCurrentScrollPos(obj)
{
    var scrollHeight = parseFloat(document.getElementById(obj).scrollHeight);
    myspeed += speed;
    
    if(myspeed >= scrollHeight)
        myspeed = scrollHeight - 1;
    else if(myspeed <= 1)
        myspeed = 1;
     
     return myspeed;
}

function onTopScroll(obj) 
{
  myspeed -= speed;
  if(myspeed <= 1)
        myspeed = 1;
        
  document.getElementById(obj).scrollTop = myspeed;
}

function onDownScroll(obj)
{ 
   document.getElementById(obj).scrollTop = getCurrentScrollPos(obj);
}