Programmable memory for an encoding system

An identification system includes a reader and a transponder. The reader initially transmits a pulse to the transponder. The transponder responds to this pulse by transmitting to the reader a sequence of a particular number (e. g. 10) of signals at a frequency dependent upon the identifying code recorded in the transponder. The reader then transmits a pulse of an opposite polarity to that previously transmitted. The transponder responds by generating 10 signals at a second frequency different from the first frequency. In this way, sequences of pulses alternatively of opposite polarity are transmitted by the reader and sequences of signals at the first and second frequencies are transmitted by the transponder. The reader decodes the sequences of signals at the first and second frequencies, in accordance with the order of the frequencies of such signals, to identify the code at the transponder. The transponder may have a programmable memory to store the code. To program the memory with such a code, the reader generates, and transmits to the transponder, sequences of pulses, each sequence coding for a programmable item of information (e. g. binary "1", binary "0" and reset). Depending upon the individual programmable item, the coded sequences may have pulses of the same polarity or of opposite polarity. At a time related to each such sequence, the reader produces a sequence different from the coded sequence to indicate that pulses coding for a programmable item follow. The transponder demodulates and decodes the transmitted pulses to recover the identifying code. This code is then recorded in the programmable memory and is used thereafter to identify the transponder when the reader interrogates the transponder as described above.