Abstract: A portable, hand-held microprocessor-based terminal forms the heart of an expandable data communication system having an architecture configured so that a variety of peripheral devices can communicate directly with the microprocessor in the portable terminal. Access to the microprocessor and associated on-board memory is made by way of an external connector coupled directly to the internal bus structure of the terminal. Data may be collected, for example, by way of inputs to a keyboard on the terminal or an optional sensor module detachably connected to the bus connector on the terminal housing. An expansion unit takes the form of a base adapted to support the portable terminal and to make connections between the terminal bus connector and peripheral devices plugged into the base so that they can communicate with the terminal microprocessor via the internal bus.