Hola.
Prueba esto.
<html>
<head>
<script>
//Por Carlos.
function calcula()
{
cadena="TRWAGMYFPDXBNJZSQVHLCKET"
posicion = formulario.dni.value % 23
letra = cadena.substring(posicion,posicion+1)
formulario.let.value=letra
}
</script>
</head>
<body>
<form name="formulario">
<p>DNI:<br>
<input type="text" name="dni" maxlenght="9" size="9">
<input type="text" name="let" size="1" readonly>
<input type="button" value="Ok" language="JavaScript" onclick="calcula()">
</form>
</body>
</html>
Un saludo. <IMG SRC="http://www.sfn.saskatoon.sk.ca/~ad676/moon/img/wink.gif"
<B>Carlos - Zaragoza</B>