Sony PCG-XG28 Primary User Manual page 80

To connect both a mouse and keyboard at the same time, use the optional
PS/2 division connector. If the wheel function of the PS/2 mouse does
not operate, connect the mouse to the serial port using the PS/2 serial
adapter that is supplied with your mouse. You can also use one of both of
the USB ports to connect a mouse and keyboard at the same time.