A mouse that connects to a computer via an expansion board. Another type of mouse is a serial mouse, which connects to a serial port. Serial mice are easier to install, but the advantage of bus mice is that they do not use up the serial port, so you can use the port for a different device (a modem, for example). A PS/2 mouse solves this problem by plugging into a special PS/2 port dedicated to the mouse. Bus mice are now obsolete.