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Basic Logic Gates
12 questions By Tony R. Kuphaldt
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Question 10 of 12
Many types of logic gate circuits are built with more than two inputs. These are useful, even necessary, in some digital circuit applications. Research the part numbers and datasheets of the following logic gate integrated circuits:
- Triple 3-input NOR gate
- Dual 4-input AND gate
- Single 8-input NAND gate
Reveal answerIt would be far too easy for you to look here for the answers! Research the website of a digital IC manufacturer, or consult a reference book, to find the part numbers and datasheets you’re looking for. Be prepared to show at least one datasheet for class discussion.
Notes:If you haven’t already, ask your students to provide a sample truth table for any of these gates.
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Question 11 of 12
Complete the truth table for a three-input AND gate:
A B C Output
0 0 0
0 0 1
0 1 0
0 1 1
1 0 0
1 0 1
1 1 0
1 1 1
Reveal answer
A B C Output
0 0 0 0
0 0 1 0
0 1 0 0
0 1 1 0
1 0 0 0
1 0 1 0
1 1 0 0
1 1 1 1
Follow-up question: how do you suppose the truth tables would look like for three-input OR, NOR, and NAND gates? Explain how one may transition from the regular two-input gates to gate circuits with more than two inputs. What remains the same despite additional input lines?Notes:There isn’t much to comment on here, but this is a concept some students do not immediately see (how gates work with more than two inputs).
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Question 12 of 12
Suppose you needed a two-input AND gate, but happened to have an unused 3-input AND gate in one of the integrated circuits (“chips”) already in the system you were building. Of course, you could just add another IC containing 2-input AND gates, but it seems a shame to waste the 3-input gate already there.
Explain what you would need to do with the third input terminal on this gate in order to use it as a 2-input AND gate:
Now, explain what to do with each of the following gates’ third inputs, in order to use each of them as 2-input gates:
In each case, describe why your solution works.
Reveal answer
Notes:A helpful way for students to think about this question is to consider gates in terms of what input state forces the output to go to a particular state. For example, AND gate outputs are forced low by any low input, therefore the unused input had better not be tied to ground - the only remaining option is to tie it high.
