A portable computer is constructed to utilize bottom add-on expansion modules, battery tray compartment plug-in modules and mass storage plug-in drawer modules. The modules permit the operating capabilities of the portable computer to be readily altered as desired or as needed at a given location or for a particular job function and do not require redesign or alteration of the internal components of the portable computer as modules are added or changed. Each module construction includes direct plug-in connectors which permit the add-on module to be directly connected to the computer bus without any cabling between the computer and the added module. The construction of the add-on modules permits the computer to be operated as a stand alone computer without any add-on modules and permits the computer to be operated as a portable computer powered by a battery. The construction also permits a wide variety of function cards, mass storage devices, communication devices, interface display devices, interface to instrumentation interfaces, interfaces to small servers (and any other part that can pick up the computer bus) to be readily associated with the portable computer.