An applet will not naturally have permission to access the serial port.
Actually, I'm pretty sure there's nothing in the JVM for accessing the
serial port at all--you'll need some kind of external library /dll and JNI
wrapper to do that.  You'll have to sign the applet and the user will have
to grant permissions to load and run the external library (presuming it's
possible at all.) Alternatively, you can do it via an applicaton or by Java
WebStart, which can eliminate some of the hassle.

Matt Humphrey http://www.iviz.com/

If you don't need to do bidirectional communications and you are using a
Windows OS then it should be fairly easy.  If not I think you are going
to find it very difficult.  The serial comm software to interface with
Java requires installation of a DLL on the client machine.  It is
disheartening that serial I/O through RS-232 and USB are not part of the
Java language support.  It severely restricts Java's usefulness in that
area of programming.