Author: M Abo Bakar Aslam

1x8 De-Multiplexer (DEMUX)

A 1x8 De-Multiplexer (DEMUX) is a digital combinational circuit that takes a single input signal and forwards it to one of eight output lines. The selection of output is controlled by three selection lines.

1. Truth Table - 1x8 De-Multiplexer

Suppose input is Input and selection lines are S2, S1 and S0. The outputs are represented as out-0 to out-7.
Total Number of Rows = 2^(number of selection lines) = 2³ = 8
InputS2S1S0Outputs
Input000out-0 = Input, out-1 = 0, out-2 = 0, out-3 = 0, out-4 = 0, out-5 = 0, out-6 = 0, out-7 = 0
Input001out-0 = 0, out-1 = Input, out-2 = 0, out-3 = 0, out-4 = 0, out-5 = 0, out-6 = 0, out-7 = 0
Input010out-0 = 0, out-1 = 0, out-2 = Input, out-3 = 0, out-4 = 0, out-5 = 0, out-6 = 0, out-7 = 0
Input011out-0 = 0, out-1 = 0, out-2 = 0, out-3 = Input, out-4 = 0, out-5 = 0, out-6 = 0, out-7 = 0
Input100out-0 = 0, out-1 = 0, out-2 = 0, out-3 = 0, out-4 = Input, out-5 = 0, out-6 = 0, out-7 = 0
Input101out-0 = 0, out-1 = 0, out-2 = 0, out-3 = 0, out-4 = 0, out-5 = Input, out-6 = 0, out-7 = 0
Input110out-0 = 0, out-1 = 0, out-2 = 0, out-3 = 0, out-4 = 0, out-5 = 0, out-6 = Input, out-7 = 0
Input111out-0 = 0, out-1 = 0, out-2 = 0, out-3 = 0, out-4 = 0, out-5 = 0, out-6 = 0, out-7 = Input

2. Boolean Expression for Output

As there are multiple outputs, we will derive Boolean expressions for each output.

out-0 = Input S2' S1' S0'
out-1 = Input S2' S1' S0
out-2 = Input S2' S1 S0'
out-3 = Input S2' S1 S0
out-4 = Input S2 S1' S0'
out-5 = Input S2 S1' S0
out-6 = Input S2 S1 S0'
out-7 = Input S2 S1 S0

3. Circuit Diagram

a. Block Diagram Representation

De-Multiplexer is commonly represented using a block diagram.

1x8 De-Multiplexer Block Diagram - visualized by edu2skill.com
Figure 1: 1x8 De-Multiplexer Block Diagram

b. Using Basic Gates

The circuit diagram of 1x8 DEMUX can be implemented using AND and NOT gates.

Circuit Diagram for 1x8 DEMUX using basic gates - visualized by edu2skill.com
Figure 2: Circuit Diagram for 1x8 DEMUX using basic gates

Circuit becomes more complex as we move from 1x2 to 1x8 DEMUX So we need to build DEMUX using some other logic i.e., 1x8 DEMUX using SMALL DEMUX.

Building 1x8 DEMUX using 1x2 DEMUX

Block Diagram for 1x8 DEMUX using 1x2 DEMUX only - visualized by edu2skill.com
Figure 3: Block Diagram for 1x8 DEMUX using 1x2 DEMUX only

Building 1x8 DEMUX using both 1x2 and 1x4 DEMUX

Block Diagram for 1x8 DEMUX using both 1x2 and 1x4 DEMUX - visualized by edu2skill.com
Figure 4: Block Diagram for 1x8 DEMUX using both 1x2 and 1x4 DEMUX