why this AND gate has three inputs - Page 1
Forum section: electronics / beginners

author: Nadiyar
author_rank: Newbie
timestamp: November 15, 2024, 11:28:34 am
content: Hi everyone, I'm new to electronics and I was trying to learn about multiplexers reading this article:https://www.electronics-tutorials.ws/combination/comb_2.htmlAt some point it is illustrating a 4 to 1 multiplexer built using NAND logic gates, but as it can be seen in the image each NAND is getting 3 inputs.May someone explain what is going on here? Shouldn't NAND gates have only 2 inputs?

author: wraper
author_rank: Unknown
timestamp: November 15, 2024, 11:34:15 am
content: QuoteWhy would it? It can have as many inputs as you want.

author: Zero999
author_rank: Super Contributor
timestamp: November 15, 2024, 11:45:13 am
content: Indeed. There are plenty of NAND gates with more than two inputs. For example, the 74HC20 is an IC consisting of two NAND gates, each having four inputs.https://www.ti.com/lit/ds/symlink/cd74hc20.pdf

author: TERRA Operative
author_rank: Super Contributor
timestamp: November 15, 2024, 11:51:21 am
content: Check out the 4068...8-input NAND gate.

author: brucehoult
author_rank: Super Contributor
timestamp: November 15, 2024, 12:03:50 pm
content: Incidentally, the Cray 1 supercomputer in 1975 was famously entirely build from only four kinds of chips:- a fast ECL chip with one 4-input NOR gate and one 5-input NOR gate- a slow/cheap chip with one 4-input NOR gate and one 5-input NOR gate- a small / fast / expensive RAM chip used for registers etc- a large / cheap / slow RAM chip

author: newbrain
author_rank: Super Contributor
timestamp: November 15, 2024, 12:17:59 pm
content: Quote from: Nadiyar on November 15, 2024, 11:28:34 amOthers have already answered about the possibility of having NAND gates with any number of inputs (BTW, did you notice? there's a 4 input NAND too!)If you want to better understand how the multiplexer works, I would advise to expand the provided truth table with:1. The inputs of each 3-input gates.2. The inputs of the final 4-inputs gate.for a total of 6 additional columns.Remember that the generic NAND gate truth table gives a 0 output only if all its inputs are at 1, and a 1 for all other combinations.

author: Terry Bites
author_rank: Super Contributor
timestamp: November 15, 2024, 01:54:17 pm
content: Worth noting is that the output NAND gate is performing an OR function. Lo can be as true as Hi!This page is greathttps://www.eeweb.com/positive-negative-and-assertion-level-logic/See the part on DeMorgan equivalents of Boolean equations.also reccomended:https://www.eeweb.com/implementing-logic-functions-using-only-nand-or-nor-gates/

author: Harry_22
author_rank: Frequent Contributor
timestamp: November 15, 2024, 08:29:42 pm
content: Hi Nadiyar!You can play with this circuit.Please see it in attachments.Here you will find freeware Microcap. Just unzip it into the folder and run mc12.https://drive.google.com/file/d/1MvPEWrPnJActRC2H1dEmYp3nr6EGssbp/view?usp=drive_link

author: wasedadoc
author_rank: Super Contributor
timestamp: November 15, 2024, 09:07:59 pm
content: In the same vein, OR and NOR gates are not limited to 2 inputs.

author: TimFox
author_rank: Super Contributor
timestamp: November 15, 2024, 09:48:37 pm
content: The Boolean (or logical) operators AND and OR are both associative and commutative.Therefore, NAND and NOR are also both associative and commutative.Not all operators are either or both.[Edit: removed second line]

author: brucehoult
author_rank: Super Contributor
timestamp: November 15, 2024, 11:05:20 pm
content: Quote from: TimFox on November 15, 2024, 09:48:37 pmThat's not true.NAND(NAND(A, B), C)is not the same asNAND(A, NAND(B, C))

author: TimFox
author_rank: Super Contributor
timestamp: November 15, 2024, 11:33:58 pm
content: I stand corrected, but it is true for AND and OR.(I have edited my post accordingly.)

author: newbrain
author_rank: Super Contributor
timestamp: November 18, 2024, 02:57:50 pm
content: Quote from: Terry Bites on November 15, 2024, 01:54:17 pmI still remember this from aFairchild TTL MSI application bookI bought when I wasn't even a teenager(page 97, 5-5):Quote