Abstract: 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.