Read the following description concerning a logic circuit, and then answer Subquestions 1 and 2
Q1. Read the following description concerning a logic circuit, and then answer Subquestions 1 and 2.
Figure 1 shows a logic circuit that is used as a control system for an electronic device. The logic circuit has three inputs (X, Y and Z) and one output (F). As shown in Figure 1, the logic circuit has a feed back line, by which the last output F is fed back and is used as the current input to the NAND gate. Therefore, the current output F is determined by three current inputs X, Y and Z and the last output F. In this control system, among three inputs X, Y and Z, at most only one input can change its value on each clock cycle.
(Feed back line)
Figure 1 The logic circuit
Meanings of the logic gate symbols used in Figure 1 are as follows:
AND gate NAND gate OR gate NOR gate
Figure 2 shows sample input/output status of the logic circuit. For example, the output F on the clock cycle c (value 0) is obtained from the inputs X, Y and Z on the clock cycle c (values 1, 0 and 1 respectively) and the output F at the clock cycle c-1 (value 0).
Clock cycle | c - 1 | c | c + 1 | …
Figure 2 Sample input/output status of the logic circuit
Subquestion 1
From the answer group below, select the correct answer to be inserted into each blank
_______ in the following three figures. If necessary, select the same answer twice or more.
Answer group
Subquestion 2
From the answer group below, select the correct answer to be inserted into the blank _______ in the following description.
When the value 1 is obtained on the clock cycle c at the intermediate point T in Figure 1, it is always true that ____D____.
Answer group
- the output F on the clock cycle c-1 was 0
- the output F on the clock cycle c-1 was 1
- both of inputs X and Y on the clock cycle c are 0
- both of inputs X and Y on the clock cycle c are 1
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