Discussion
The output of a two-input NAND gate is LOW only when
Answer: C
Correct answer: both inputs are HIGH
A NAND gate is an AND gate followed by an inverter, so its output is the complement of AND. AND gives 1 only when both inputs are 1; NAND therefore gives 0 only in that single case and 1 in the other three. Its truth table is worth holding in mind: 0,0 gives 1; 0,1 gives 1; 1,0 gives 1; 1,1 gives 0. NAND is called a universal gate because every other logic function can be built from it alone. An inverter is a NAND with both inputs tied together; an AND is a NAND followed by another NAND acting as an inverter; an OR is a NAND fed by two inverting NANDs, which is De Morgan’s theorem in hardware. NOR is universal for the same reason. That property matters in practice, since a manufacturer can build an entire logic family from one repeated cell.
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