A method and apparatus for transmitting data in a radio frequency identification system, which includes a reader and at least one transponder, is provided. The reader transmits data sequences to the transponder including at least command data, associated check data, and a subsequent return link header data. Upon receiving unrecognized command data and/or unsupported parameter and address data the transponder notifies the reader of this condition. The notification takes place at times when the reader transmits end signals indicating an end of a data sequence, transmits an optional parameter and address data, or transmits the return link header data. Thus, communication between the reader and other transponders is not disrupted when a transponder cannot execute a command.