Security keyboard unit, in particular for the composition of a confidential code

摘要

A 16-key keypad is arranged in matrix fashion in four rows (a, b, c, d) and four columns (1, 2, 3, 4). Interrogation pulses are applied cyclically in sequence to the four rows (a, b, c, d). In a state in which the keypad is being explored, the microprocessor also applies the interrogation pulse to all the columns except one. In the event of the non-transference of the interrogation pulse to the column concerned, it immediately applies to the column a dummy pulse which is terminated with the interrogation pulse. Every now and then, in a state of pure simulation, the microprocessor also applies the interrogation pulse to one of the rows and all the columns of the keypad. The signals obtained in this way on the rows and columns are then indistinguishable from those corresponding to an actual pressing of a key. IMAGE