The serial port Version isn't released yet.

It'll look the same as the parallel port version, but is more difficult to build because it'll use a microcontroller to communicate with the PC's serial port.

Why then a serial version ?
The serial version has one main advantage.
You can use much longer cables to connect the serial version to the PC. That makes the serial version ideal for car use, where the player hardware is in the back and the interface in the front of the car.