Abstract: A reader interrogates an object. The object has a tag with a data source such as a read-only memory (ROM) which produces an identifying sequence of binary 1's and 0's in an individual pattern and transmits these signals to the reader. Each "1" or "0" is converted to a plurality of signal cycles at first and second harmonic frequencies. For example, a "1" may be identified by two signal cycles at the second frequency (e.g. 40 kHz) and then one signal cycle at the first frequency (e.g. 20 kHz) and a "0" may be identified by a signal cycle at the first frequency and then two signal cycles at the second frequency. The transponder also produces a plurality of signal cycles in an individual code (different from any combination of "1's" and "0's") to indicate the end of the transmission of the signal cycles identifying the object and the start of another transmission.