To open a security door, the code 2 7 2 0 has to be entered on a keypad. The numbers are entered
one by one. The user may press arbitrary keys (0. . . 9) on the keypad; on an erroneous input, a warning
signal W should be given (W = 0: no erroneous input; W = 1: erroneous input). On entering the
next digit, the signal W is set to 1 or 0 again, depending on the former correctness of the current input
(including the new digit). When the code 2 7 2 0 is entered completely in the right order, the lock
should be opened by setting the signal E = 1. Note that there may be digits which when entered make
the current input erroneous but which also belong to the correct code. This correct code needs to be
recognized, too. Additionally the signal W needs to be set to 1 on entering the digit which makes the
current input erronenous. After that, processing may continue, considering the past input. The door
closes and locks itself automatically after use, until a correct code unlocks it again. Design a sequential
circuit for controlling the lock:
(a) Identify the inputs and outputs of the automat. Develop a wise coding for the inputs (the digits
of the keypad) to keep the expense small. If a combinational circuit is needed for the coding, also
state it (with drawing).
(b) Draw the complete Automatengraph
(c) Give the corresponding truth table and minimize the output functions and state transitions
(d) Draw the corresponding sequential circuit of the automat as FPLA using D-Filp flops.
one by one. The user may press arbitrary keys (0. . . 9) on the keypad; on an erroneous input, a warning
signal W should be given (W = 0: no erroneous input; W = 1: erroneous input). On entering the
next digit, the signal W is set to 1 or 0 again, depending on the former correctness of the current input
(including the new digit). When the code 2 7 2 0 is entered completely in the right order, the lock
should be opened by setting the signal E = 1. Note that there may be digits which when entered make
the current input erroneous but which also belong to the correct code. This correct code needs to be
recognized, too. Additionally the signal W needs to be set to 1 on entering the digit which makes the
current input erronenous. After that, processing may continue, considering the past input. The door
closes and locks itself automatically after use, until a correct code unlocks it again. Design a sequential
circuit for controlling the lock:
(a) Identify the inputs and outputs of the automat. Develop a wise coding for the inputs (the digits
of the keypad) to keep the expense small. If a combinational circuit is needed for the coding, also
state it (with drawing).
(b) Draw the complete Automatengraph
(c) Give the corresponding truth table and minimize the output functions and state transitions
(d) Draw the corresponding sequential circuit of the automat as FPLA using D-Filp flops.
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