Onkeydown event on a <asp:textbox>-how to display the value

[GP]

On the keydown event of a webcontrol in asp.net,I want to display the total
number of characters typed in another text box,how do I do it?
I have a text box web control,I want to check the length of the character on
the key down event of the textbox control which is a client side validation.I
am able to trap the keydown event the control displays an alert message "Key
down fired".
I am passing the control itself as parameter like
onkeydown="chkMaxLength(this)")
In the java script function
function chkMaxLength(input)
{
var element;
element=document.getElementById(input.name);
alert(element.Value);
}
It always gives the error at document.getElementById(input.name),can some
one help how to retreive the length and display it in another text box
control.
Thanks
GP