A hand-held data processing system of modular structure includes a base unit that couples with a reader unit as well as other units. The other units can include display, printing, communication or other interface functionality. The base unit can also function when separate from the reader unit and the other units. The base unit can include an indicia reader or a display component. In addition, the base unit, reader unit and the other units can possess the capability of coupling with an accessory module such as a memory device, processor or microcomputer. The reader unit and the other units can include power supplies that can supply power to the base unit. Finally, a combined assembly of a base unit and a reader or other unit can be of a size that fits within a shirt pocket.