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logic gate (AND, OR, XOR, NOT, NAND, NOR and XNOR)

Contributor(s): Margaret Rouse

A logic gate is a building block of a digital circuit. Most logic gates have two inputs and one output and are based on Boolean algebra. At any given moment, every terminal is in one of the two binary conditions false (high) or true (low). False represents 0, and true represents 1. Depending on the type of logic gate being used and the combination of inputs, the binary output will differ.  A logic gate can be thought of like a light switch, wherein one position the output is off—0, and in another, it is on—1.  Logic gates are commonly used in integrated circuits (IC).

Basic logic gates

There are seven basic logic gates: AND, OR, XOR, NOT, NAND, NOR, and XNOR.

AND | OR | XOR | NOT | NAND | NOR | XNOR

The AND gate is so named because, if 0 is called "false" and 1 is called "true," the gate acts in the same way as the logical "and" operator. The following illustration and table show the circuit symbol and logic combinations for an AND gate. (In the symbol, the input terminals are at left and the output terminal is at right.) The output is "true" when both inputs are "true." Otherwise, the output is "false." In other words, the output is 1 only when both inputs one AND two are 1.

/WhatIs/images/and.gif (220 bytes)

AND gate

Input 1 Input 2 Output
     
  1  
1    
1 1 1

The OR gate gets its name from the fact that it behaves after the fashion of the logical inclusive "or." The output is "true" if either or both of the inputs are "true." If both inputs are "false," then the output is "false." In other words, for the output to be 1, at least input one OR two must be 1.

/WhatIs/images/or.gif (224 bytes)

OR gate


Input 1 Input 2 Output
     
  1 1
1   1
1 1 1

 

The XOR ( exclusive-OR ) gate acts in the same way as the logical "either/or." The output is "true" if either, but not both, of the inputs are "true." The output is "false" if both inputs are "false" or if both inputs are "true." Another way of looking at this circuit is to observe that the output is 1 if the inputs are different, but 0 if the inputs are the same. 

 

XOR gate

Input 1 Input 2 Output
     
  1 1
1   1
1 1  

 

A logical inverter, sometimes called a NOT gate to differentiate it from other types of electronic inverter devices, has only one input. It reverses the logic state. If the input is 1, then the output is 0. If the input is 0, then the output is 1.  

 

/WhatIs/images/not.gif (240 bytes)

 

Inverter or NOT gate
Input Output
1  
  1

 

The NAND gate operates as an AND gate followed by a NOT gate. It acts in the manner of the logical operation "and" followed by negation. The output is "false" if both inputs are "true." Otherwise, the output is "true."

/WhatIs/images/nand.gif (240 bytes)

NAND gate

Input 1 Input 2 Output
    1
  1 1
1   1
1 1  

 

The NOR gate is a combination OR gate followed by an inverter. Its output is "true" if both inputs are "false." Otherwise, the output is "false."

/WhatIs/images/nor.gif (237 bytes)

NOR gate

Input 1 Input 2 Output
    1
  1  
1    
1 1  

 

The XNOR (exclusive-NOR) gate is a combination XOR gate followed by an inverter. Its output is "true" if the inputs are the same, and"false" if the inputs are different.

/WhatIs/images/xnor.gif (278 bytes)

XNOR gate

Input 1 Input 2 Output
    1
  1  
1    
1 1 1

Using combinations of logic gates, complex operations can be performed. In theory, there is no limit to the number of gates that can be arrayed together in a single device. But in practice, there is a limit to the number of gates that can be packed into a given physical space. Arrays of logic gates are found in digital ICs. As IC technology advances, the required physical volume for each individual logic gate decreases and digital devices of the same or smaller size become capable of performing ever-more-complicated operations at ever-increasing speeds.

Composition of logic gates

High or low binary conditions are represented by different voltage levels. The logic state of a terminal can, and generally does, change often as the circuit processes data. In most logic gates, the low state is approximately zero volts (0 V), while the high state is approximately five volts positive (+5 V).

Logic gates can be made of resistors and transistors, or diodes. A resistor can commonly be used as a pull-up or pull-down resistor. Pull-up or pull-down resistors are used when there are any unused logic gate inputs to connect to either a logic level 1 or 0 respectively. This prevents any false switching of the gate. Pull-up resistors are connected to Vcc (+5V), and pull-down resistors are connected to ground (0 V).

Commonly used logic gates are TTL and CMOS. TTL, or Transistor-Transistor Logic, ICs will use NPN and PNP type Bipolar Junction Transistors. CMOS, or Complementary Metal-Oxide-Silicon, ICs are constructed from MOSFET or JFET type Field Effect Transistors. TTL IC’s may commonly be labeled as the 7400 series of chips, while CMOS ICs may often be marked as a 4000 series of chips.

See an introductory tutorial on logic gates:

This was last updated in July 2019

Continue Reading About logic gate (AND, OR, XOR, NOT, NAND, NOR and XNOR)

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there are only three basic gates AND OR NOT
Universal Gates are NAND NOR
Derrived gates are XOR XNOR
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What do thou mean good sir?
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well that only depends on if there would be no new inventions

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it is not given a full part just basic is here
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WHY ARE THE NAMES OF LOGIC GATES ALL CAPITAL LETTERS??
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Because if i was to call an and gate an and gate or an or gate an or gate it would look confusing
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i still don't get it! :(
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Easy, a great refresher. This is basic. The diagrams help the most.
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can you post the gate transformation next time.

but thanks a lot,
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where is the memory cell gate?
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its nice ,i like it
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Thank you! Only thing I found that actually makes sense.
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This is very useful...
William
Chary
Rachel VO
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Very cool
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Please note that there are no 7 basic gates but there are ONLY and ONLY 2 basic gates that are AND and OR and an INVERTER with using AND, OR and INVERTOR you can create any type of gate, perhaps author was confused with 7 colors of white lights, but digital binary world is a way different than the Analog business.
Thanks,
Mansoor Alam
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what about the exclusive OR

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What type of logic gates did you know of previously?
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Thanks, man. I just needed some clearing up on OR and XOR for some code I'm working on!
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Or is OR an OR XOR but ORXOR is an OR variant of XOR, very cool!
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I am in Middle school and my dad is teaching CS50, a Harvard coding class at a high school, I saw him go over different gates so I build each one in the game Minecraft using Redstone, basically wires.
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Much better. Thanks!
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you saved my day thanks guys....

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If you use a  ~  it will invert your BIT like a NOT gate. example IN ~BI01 will be inverted
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Yeah just go into your BIOS and press 'Reset your PC'
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This is very simple and understandable.
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Simple....like you
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very educative and easy to understand thanks
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Many thanks, I would like to open your dog's arseholoe
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Absolutely wonderful.
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Indeed, also I penetrated a moose
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Thanks i understand this now
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i am not understanding logic gate sir
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aha me neither but go with the vibe 
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yEaH bRo ToTaLlY gO wItH tHe ViBe MaN....
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9=10=q59083495659
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is there a situation where the out put of a logic gate become two?
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sir my question is that
Where Logical Gates Are Used in Computer architecture.
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i like trains
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Baptism in the name of the Father, AND the Son, AND the Holy Spirit.
The output is "true" if all of the inputs are "true."
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