# How to use a 8-bit multiplexer

#### yjpang84

Joined Sep 10, 2006
1
Guys im desperate..

this is the qn: Z is a boolean function of 3 input variables A,B and C. If A=C; else Z=0.

Design and draw a clearly labelled diagram to implement Z using only one 8-bit multiplexer.

i don't even understand the question. please enlighten.

#### thingmaker3

Joined May 16, 2005
5,084
What happened to "then?" Is the "then" supposed to be "Z=1?" Is B a "red herring" variable?

I would contact the instructor to see if there is a typo.

#### rishid

Joined Oct 24, 2005
8
Truth table for this is:
A B C | Z
0 0 0 | 1
0 0 1 | 0
0 1 0 | 1
0 1 1 | 0
1 0 0 | 0
1 0 1 | 1
1 1 0 | 0
1 1 1 | 1

Then I think you just use A,B,C as the select bits in the 8bit mux and draw either 1 or 0's as in puts depedning on the value of Z for the specific mux input

#### hgmjr

Joined Jan 28, 2005
9,029
I think you are going to need to incorporate a 4-input "OR" gate into which you will need to feed the appropriate outputs of the mux to complete the implementation.

hgmjr

#### Papabravo

Joined Feb 24, 2006
16,828
No need for the 4-input gate. The single output of the mux is all you need. The select inputs A, B, and C "select" one of the constant inputs to route to the output. It could hardly be simpler.

#### hgmjr

Joined Jan 28, 2005
9,029
My bad. Papabravo is correct. I was thinking of 3-in / 1-of-8 out decoder when I recommended an OR gate.

hgmjr

#### Papabravo

Joined Feb 24, 2006
16,828
It's an easy mistake to make without a diagram. Although I have known about this technique since I was an undergraduate, I have never had occasion to use it in a career that has spanned nearly four decades.