# 3 input NAND Gates for function

Discussion in 'General Electronics Chat' started by Adriana, Nov 15, 2014.

Nov 15, 2014
2
0
Hi all!

I have a 74HC10 component with 3 input NAND-gates and I am trying to use it to connect a function which is

X="overline/inversion of"(AB)

I am not sure what to do with the third input in the NAND-gate.

2. ### djsfantasi AAC Fanatic!

Apr 11, 2010
2,815
835
Is the function you describe ("X="overline/inversion of"(AB)") look familiar? It's a common logic gate…

As far as your question about an unused input, draw out the truth table for a three input NAND gate. Look at the output if one input is tied low. Is it the same as the truth table for a two input NAND? Repeat the exercise if one input is held high. The result of this exercise will show you what to do with the unused input.

3. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,399
497
(AB)' isn't that De Morgan's Law?

Nov 15, 2014
2
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Sorry Im not really following, could you please elaborate?
I have done the truth table, but still dont get it..

Btw yes X=(AB)' would have beena better way to express the problem statement

5. ### djsfantasi AAC Fanatic!

Apr 11, 2010
2,815
835
Here is the truth table for a 2 input NAND gate, which is (AB)'

And for a 3 input NAND gate

Now take a look only at the portion of the table where A is low. Do the columns B, C and Y look like a 2 input NAND gate? Do the same exercise for the portion of the table where A is high (A=1)?

What value should you tie an input of the unused pin of a 3 input NAND, for the remaining pins to act like a 2 input NAND gate?