Abstract: A module for interfacing with a serial data bus of an on-board computer system of a motor vehicle to provide a means by which an external device such as a lap top computer, electronic compass, etc., can exchange useful data therefrom. The module includes a controller having a vehicle data bus interface circuit which interfaces the module with the serial data bus. An accessory device interface circuit interfaces the module with the external device. The module provides a power connection terminal to provide power from the vehicle's electrical system to the external device. The module forms a low cost means by which various external devices can be interfaced with the on-board computer of the vehicle to exchange useful information and to enhance operation of the vehicle or the external device.