Abstract: A transponder, comprising means for receiving a periodically modulated interrogating signal; means for demodulating the received signal to produce a periodic baseband signal; and means for transmitting data at a rate determined by the periodicity of the baseband signal.

Abstract: A memory suitable for use as a responder, comprising:(a) receiver means for receiving a transmitted signal S1;(b) generator means (76) for generating a carrier component of a transmittable signal S2;(c) logic means (L.sub.2) for enabling signal S2 to be in mathematical relationship with signal S1, said logic means comprising:(i) at least one memory means (60) for storing code;(ii) at least one modulation means (65) for modulating said carrier component of signal S2, the modulating being under control of at least a portion of said stored code, such that signal S2 is in coded mathematical relationship with signal S1; and(d) transmitter means (T.sub.2) for transmitting signal S2.