function CheckFieldLength(textbox, counterfield, maxlength) 
{
    var counterfield = document.getElementById(counterfield);
    var len = textbox.value.length;
    if (len > maxlength) 
    {
        textbox.value = textbox.value.substring(0,maxlength);
        len = maxlength;
    }
    
    counterfield.innerHTML = maxlength - len;
}