| 摘要 |
A circuit produces a control signal which is input through a bi-directional communication device to a removable circuit. The removable circuit is set to a first logic state which is a function of the control signal and thereafter steps through a sequence of logic states with each logic state being a function of the previous logic state. The entire sequence of logic states is represented by a state signal or code word which is a function of the control signal. The produced state signal is compared to a predetermined stored state signal. Access is enabled in response to the production of a proper sequence of logic states, as indicated by the comparison of the produced state signal to the predetermined stored state signal. Security is improved because without knowledge of the control signal input to the removable circuit, an unauthorized user cannot ascertain the code word.
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