A portable field maintenance tool system for performing a plurality of different functional tasks on one or more equipment assets in a plant includes a handheld base and a plurality of functional modules for attachment to the handheld base. Each functional module, when operatively attached to the handheld base, interacts with computer circuits on the handheld base to perform a pre-defined one or more of the functional tasks. A portable handheld field maintenance tool for performing one or more pre-selected ones of the functional tasks on an equipment asset in a plant is formed by operatively mounting any one of the functional modules to the handheld base.