A modular test instrument comprises a base unit and one of a selection of application modules, neither being capable of performing end-user functions without the other. When physically assembled to one another, the base unit and application module comprise a structurally unitary device specialized for performance of application-specific end-user functions. The base unit is generic to all types of test to be provided, and comprises a display, a power supply, a user interface, and generic software to operate the display and user interface. Each application module is connected to the base unit in the same manner, so that the same base unit can he employed with a wide variety of application modules. The application modules can be readily and conveniently removed from and assembled to the base unit. The application-specific application modules each include a physical interface for establishing signal-communicating connection to the equipment to be tested, and application-specific program data and software, including information required to provide appropriate test signals, test messages, and the like. Each application module also stores user interface software for providing an application-specific user interface. When an application module is assembled to a base unit, polling software comprised by the base unit determines the identity of the application module, and determines whether it stores any software or other program data not previously copied to the base unit's memory; if not, the copying step is begun automatically upon assembly.