var max_char;

// set_text = function(a)

function set_text(a)
{
    max_char = a;
    run_func();
}

function run_func()
{
  if (document.getElementById('comments'))
  {
    limitText(document.getElementById('comments'),max_char,document.getElementById('charcnt'));
  }
}

function limitText(limitField,limitNum,dispCharLeft)
{
  if (limitField.value.length > limitNum )
  {
    limitField.value = limitField.value.substring(0, limitNum);
  }
  else
  {
    dispCharLeft.innerHTML = limitNum - limitField.value.length + "";
  }
}

document.onkeyup = run_func;