# How multiplexer truth table works?

#### Doraemon

Joined May 21, 2015
13

The truth table is:

The result is D I guess. And according to the diagram, the logic is ((I1 AND A) OR (I0 AND A)) I guess.
I'm new to electronics and this truth table is confusing, anyone can help?
How the calculation is carried out?

#### kubeek

Joined Sep 20, 2005
5,785
I think the correct equation is ((I0 AND A) OR (I1 AND (NOT A)))

#### tshuck

Joined Oct 18, 2012
3,534
I would argue that the truth table is wrong - or, at best, misleading.

Row 4 seems to suggest that when the select (A) is 0, the output (D) is input 1 (I1).

A 2-to1 multiplexer works according to the following: When the select is 0, the output is the value of input 0, the output is the same as input 1, otherwise.

This means the equation is $$D=I_{1}*select+I_{0}*\overline{select}$$

#### Doraemon

Joined May 21, 2015
13
Thank you I was trying to figure this out since last 2 hours XD.

#### WBahn

Joined Mar 31, 2012
28,505
While the truth table is that of a 2:1 MUX, the naming of the signals is very misleading. Also, in the e-book, the next diagram that shows the implementation is all but worthless since it doesn't include any distinction between the two outputs of the 1:2 line decoder.

A much better presentation of the 2:1 MUX truth table, in my opinion, is to put the select lines to the left of the table.

Code:
S | A1 | A0 || Y
--+----+----++---
0 |  0 |  0 || 0
0 |  0 |  1 || 1
0 |  1 |  0 || 0
0 |  1 |  1 || 1
1 |  0 |  0 || 0
1 |  0 |  1 || 0
1 |  1 |  0 || 1
1 |  1 |  1 || 1