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A radio frequency identification transponder including a power supply and a dynamic memory array which stores data. When power from the power supply ceases, the data in the dynamic memory array is validly maintained for a predetermined period of time. The dynamic memory array is responsive to an int

A radio frequency identification transponder including a power supply and a dynamic memory array which stores data. When power from the power supply ceases, the data in the dynamic memory array is validly maintained for a predetermined period of time. The dynamic memory array is responsive to an interrogating signal for selectively updating the data. A signal processor extracts an identifier from the interrogation signal and is responsive to the identifier and the stored data to determine whether some or all of the identifier is stored in the dynamic memory array.

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1. A radio-frequency identification (“RFID”) transponder the RFID transponder comprising: a receiver for receiving a plurality of temporally spaced interrogating signals from an RFID interrogator, wherein each of the plurality of temporally spaced interrogating signal includes a respective identifie

1. A radio-frequency identification (“RFID”) transponder the RFID transponder comprising: a receiver for receiving a plurality of temporally spaced interrogating signals from an RFID interrogator, wherein each of the plurality of temporally spaced interrogating signal includes a respective identifier;a memory array for storing first data, wherein the first data comprises a time stamp number of a first instance the RFID transponder was interrogated by the RFID interrogator and wherein the first data is indicative of an order of the temporally spaced interrogating signals;a signal processor that extracts the respective identifier;wherein the signal processor determines if the extracted identifier is different than the first data and if the RFID transponder was previously interrogated by the RFID interrogator;responsive to a determination that the extracted identifier is different than the first data and that the RFID transponder was previously interrogated by the RFID interrogator, the first data remains unchanged: andresponsive to a determination that the extracted identifier is different than the first data and that the RFID transponder was not previously interrogated by the RFID interrogator, the first data is overwritten. 2. The RFID transponder according to claim 1 wherein at least some of the identifier is stored in the memory array and the stored at least some of the identifier updates at least some of the first data. 3. The RFID transponder according to claim 1 wherein the signal processor determines that the identifier is stored in the memory array if the identifier is different than the first data. 4. The RFID transponder according to claim 1 wherein the signal processor determines that the identifier is stored in the memory array if a predetermined portion of the identifier is different than the first data. 5. The RFID transponder according to claim 1 wherein the identifier includes data selected from at least one of the following types: second data indicative of the interrogator; andthird data that is not constant between respective interrogating signals. 6. The RFID transponder according to claim 5 wherein the third data is indicative of timing of a respective interrogating signal. 7. The RFID transponder according to claim 5 wherein the third data is selected from the group consisting of a pseudo random number and a character string. 8. The RFID transponder according to claim 5 wherein the second data includes a reference number for the interrogator that is the same for all interrogating signals provided by the RFID interrogator. 9. The RFID transponder according to claim 8 wherein the third data includes a stamp that varies with at least one of date and time. 10. The RFID transponder according to claim 5 wherein the signal processor determines that the second data is stored in the memory array if the second data is different than the first data. 11. The RFID transponder according to claim 5 wherein the signal processor determines that the third data is stored in the memory array if the second data is different than the first data. 12. The RFID transponder according to claim 11 wherein the signal processor determines that the second data is also stored in the memory array if the second data is different than the first data. 13. The RFID transponder according to claim 5 wherein the signal processor determines that the identifier is stored in the memory array if the third data is sufficiently different than at least some of the first data. 14. The RFID transponder according to claim 13 wherein: the third data includes a stamp indicative of at least one of date and time of a respective interrogating signal; andthe stamp is compared with the first data to determine whether sufficient time has elapsed since a last store of the third data in the memory array. 15. The RFID transponder according to claim 5 wherein: the RFID transponder includes a transmitter for transmitting a reply signal to the interrogator in response to an interrogating signal; andthe reply signal includes fourth data indicative of the transponder and fifth data derived from the first data. 16. The RFID transponder according to claim 15 wherein the fifth data includes at least one of the second data and the third data. 17. The RFID transponder according to claim 15 wherein the fifth data includes the second data. 18. The RFID transponder according to claim 15 wherein the fifth data includes the third data. 19. The RFID transponder according to claim 15 wherein the fifth data includes the second data and the third data. 20. The RFID transponder according to claim 15 wherein the fourth data is a coded string for allowing discrimination between other RFID transponders used with the interrogator. 21. A method of comparing signals transmitted between a radio-frequency identification (“RFID”) interrogator and a RFID transponder with first data stored in a memory, the method comprising: transmitting, by the RFID interrogator, a plurality of temporally spaced interrogating signals, the temporally spaced interrogating signals including respective identifiers;receiving, by a receiver of the RFID transponder, the plurality of temporally spaced interrogating signals;extracting, by a signal processor of the RFID transponder, information from the received plurality of temporally spaced interrogating signals;determining, by the signal processor of the RFID transponder, if the extracted information is different than the first data and if the RFID transponder was previously interrogated by the RFID interrogator, wherein the first data is indicative of an order of the temporally spaced interrogating signals;responsive to a determination that the extracted information is different than the first data and that the RFID transponder was previously interrogated by the RFID interrogator, the first data remains unchanged; andresponsive to a determination that the extracted identifier is different than the first data and that the RFID transponder was not previously interrogated by the RFID interrogator, the first data is overwritten. 22. The method according to claim 21, wherein the information is an interrogator identifier. 23. The method according to claim 21, wherein the transponder receives the transmitted signal. 24. The method according to claim 21, wherein the interrogator receives the transmitted signal. 25. The method according to claim 21, wherein the information includes a stamp that varies with at least one of date and time. 26. The RFID transponder according to claim 1 wherein the identifier is an interrogator identifier. 27. The RFID transponder according to claim 1 wherein the signal processor further determines whether some or all of the extracted identifiers are stored in the memory array subsequent to the comparison of the extracted identifier and the first data. 28. A radio-frequency identification (“RFID”) interrogator, the RFID interrogator comprising: a transmitter for transmitting a plurality of temporally spaced interrogating signals to at least one RFID transponder, the at least one RFID transponder comprising:a receiver for receiving the plurality of temporally spaced interrogating signals from the RFID interrogator, wherein each of the plurality of temporally spaced interrogating signal includes a respective identifier;a memory array for storing first data, wherein the first data comprises a time stamp number of a first instance the RFID transponder was interrogated by the RFID interrogator and wherein the first data is indicative of an order of the temporally spaced interrogating signals;a signal processor that extracts the respective identifier; wherein the signal processor determines if the extracted identifier is different than the first data and if the RFID transponder was previously interrogated by the RFID interrogator; andresponsive to a determination that the extracted identifier is different than the first data and that the RFID transponder was previously interrogated by the RFID interrogator, the first data remains unchanged;responsive to a determination that the extracted identifier is different than the first data and that the RFID transponder was not previously interrogated by the RFID interrogator, the first data is overwritten;a receiver for receiving a response signal from the at least one RFID transponder;a signal processor that extracts identity data from the response signal;wherein the response signal is compared with communications sent to each of the at least one RFID transponder;wherein the response signal includes the identity data derived from the temporally spaced interrogating signals; andwherein the identity data is indicative of an order of the temporally spaced interrogating signals. 29. The RFID interrogator as claimed in claim 28, wherein the response signal includes a stamp that varies with at least one of date and time. 30. A radio frequency identification (RFID) system comprising: at least one RFID transponder for use with an RFID interrogator that provides a plurality of temporally spaced interrogating signals,the RFID transponder comprising:a first receiver for receiving the plurality of temporally spaced interrogating signals, wherein each of the plurality of temporally spaced interrogating signal includes a respective identifier;a memory array for storing first data, wherein the first data comprises a time stamp number of a first instance the RFID transponder was interrogated by the RFID interrogator and wherein the first data is indicative of an order of the temporally spaced interrogating signals;a first signal processor that extracts the respective identifier and determines if the extracted identifier is different than the first data and if the RFID transponder was previously interrogated by the RFID interrogator; andresponsive to a determination that the extracted identifier is different than the first data and that the RFID transponder was previously interrogated by the RFID interrogator, the first data remains unchanged;responsive to a determination that the extracted identifier is different than the first data and that the RFID transponder was not previously interrogated by the RFID interrogator, the first data is overwritten;the RFID interrogator transmits to the at least one RFID transponder, the interrogating signals, the at least one RFID transponder providing, to the RFID interrogator, upon receipt of the identifier, a response signal, the RFID interrogator comprising:a second receiver for receiving the response signal from the at least one RFID transponder;a second signal processor that extracts identity data from the response signal; andwherein the response signal is compared with communications sent to each of the at least one RFID transponder.