Abstract: A computer system includes a first circuit board and a second circuit board which is detachably coupled to the first circuit board via a specially designed I/O connector. When coupled together, the main board and the I/O card are positioned within a same plane by which, a computer system is configured with easy customizable printed circuit boards without substantially increase of the overall physical dimension. In the event that the first circuit board or the second circuit board needs to be modified, the two circuit boards can be detached from each other and after the modification, the two circuit boards can be coupled together again with the same connector to resume the functions of the computer system. Embodiments of the present invention provide effective and convenient solutions for reconfiguration, upgrading, addition and/or removal of external I/O ports in computer systems without redesigning and/or replacing the whole motherboard.