Hola DINASEN.
<input name="cp" type="text" class="campos" style="width:40px; height: 16px;" maxlength="5" onKeypress="LP_data(event) />
function LP_datatex(e{
var key=(document.all) ? e.keyCode : e.wich;
if (key < 48 || key > 57){
}
Espero que te sirva. Saludos,
