The TK5551 is a completely programmable R/W transponder which implements all important functions for identification systems, including anti-collision (e.g., 10 transponders in < 500 ms depending on the application). It allows the contactless reading and writing of data which are transmitted bi-directionally between a read/write base station and the transponder. It is a plastic-packaged device which accommodates the IDIC e5551 and also the antenna realized as an LC-circuit. No additional external power supply is necessary for the transponder because it receives power from the RF field generated by the base station. Data are transmitted by modulating the amplitude of the RF field. The TK5551 can be used to adjust and modify the ID code or any other stored data, e.g., rolling code systems. The on-chip 264-bit EEPROM (8 blocks, 33 bits per block) can be read and written block wise from the base station. The blocks can be protected against overwriting. One block is reserved for setting the operation modes of the IC. Another block can obtain a password to prevent unauthorized writing.