# Why Encoder circuits use OR logic gate and Decoder use AND?

Discussion in 'General Electronics Discussion' started by Prohor, Feb 3, 2018.

1. ### Prohor

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Sep 27, 2016
I am searching for answer why Encoder circuit uses OR logic gate to encode input to output and Decoder uses AND logical gate?

Can anyone help me with some explanation?

2. ### Alec_t

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Jul 7, 2015
Which encoder?
Which decoder?
Schematics?

3. ### Prohor

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Sep 27, 2016
Digital Encoder using logic gates ... please see this tutorial
https://www.electronics-tutorials.ws/combination/comb_4.html

I just want know in case of encoder why AND logic gate is not used and why in case of decoder OR gate not used?

4. ### Alec_t

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Jul 7, 2015
The Boolean logic will dictate which type of gate is used. In the case of the 8:3 priority encoder shown, only 3 4-input OR gates are needed. You could build the encoder using NAND gates, but I'm pretty sure you'd need a lot more gates.

Prohor likes this.
5. ### Prohor

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Sep 27, 2016
Yes, I know as a universal logic gate both NAND and NOR can do the same circuit implemented. But I am still willing to know why in encoder OR gate is used instead of AND gate? I am a simple theory of my own like: as "Encoder has higher numbers of inputs like 8 x 3 , so or gate do the job as it sums . On the other hand Decoder like 3x8 has higher numbers of output so it requres AND gate as it has common character to multiplication - So that If I use OR gate in Decoder, it will give always smaller number of outputs instead of Greater numbers of output!!!" -- Is my theory ok?

6. ### Alec_t

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Jul 7, 2015
I don't follow your theory or see how it relates to the truth table shown. How would you produce the 8:3 prority encoder using only AND gates?

7. ### Harald KappModeratorModerator

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Nov 17, 2011
The number of inputs or outputs has no relevance to the type of logic gates required to build a boolean function. Every boolean expression has an equivalent gate type to realize that expression in hardware.
boolean + = logic OR
boolean * = logic AND
booleant not = logic not

All other logic functions can be constructed from these 3 basic equivalences.