Multiplexor

A multiplexor is a device which takes in multiple signals and outputs a single signal. It works as an electronic switch.

The non-select inputs are numbered in binary, so for a multiplexor with two select inputs, the first signal (A) would be labelled 00, the second (B) labelled 01, the third (C) labelled 10 and the forth (D) labelled 11. This means if none of the select input signals is on, the first non-select input signal (A) is copied to the output. If both are on, the fourth (D) is copied across. It is normal to have the inputs labelled up using their binary labels on schematic diagrams as shown below

Inputs      X Y
| |
|\ | |
|  \ |
Input A -|00  \
Input B -|01   |
Input C -|10   |

Output M
Input D -|11  /
|  /

Rather than continue with the ascii art, i'll represent this in the following way:

Y Z

A00
B01
C10
D11         Output M

It can be noted that 2^N non-select signals require N select signals. So 8 non-select signals require 3 select signals. In the diagram below A-H are the non-select inputs with their binary code before them, X,Y and Z are the select inputs and Output M is the multiplexor result. A-H and X-Z each have a value of 0 or 1 fed into them. Depending on the values of X,Y and Z, the value of M changes to the value fed into one of A-H.

X Y Z

A000
B001
C010
D011         Output M
E100
F101
G110
H111

If X = 0, Y = 1, Z = 0 for example, M = the value fed into C.

Attempt the problem below with all numbers being the values fed into the inputs.

X Y Z
1 0 0

1D         Output M

What value is copied to M?

The correct answer is 0 because E is selected when X=1 Y=0 and Z=0 and the value fed into E is 0.

Now try X=1, Y=1, Z=0 for the diagram above

The answer is 0 again, this time from G.