What is Demultiplexer? Circuit diagram, truth table and applications

Demultiplexer or Demux is a combinational circuit that distributes the single input data to a specific output line. The control inputs or selection lines are used to select a specific output line from the possible output lines.

Demultiplexer works opposite to that of the multiplexer. Demux has one output, 2ⁿ possible outputs and n control or selection lines. It is also called a data distributor.

The demultiplexer circuit is shown in the above diagram. It has one data input(D), 2ⁿ possible outputs(Y₀, Y₁, Y₂,…Y₂^n-1), n selection lines(S₀, S₁,…S_n). It also has an enable input. The demux will work only when the enable is set to logic 1.

As like multiplexer, the demux also has several types based on the number of possible outputs. It includes 1-to-4 demux, 1-to-8 demux, etc.

Types of Demultiplexer

1 : 4 demultiplexer

The block diagram and circuit of 1-to-4 demultiplexer are shown below. There are four possible outputs Y₀, Y₁, Y₂, Y₃ and a single input D. The single data input is sent to one of the four outputs as per the selection line input.

If the selection line input S₁S₀ = 00, the first AND gate in the above circuit diagram gets enabled. It is because both the AND gate receives the inverted input. Since all the remaining AND gates get 0 from the S₁S₀ at any one of the inputs, they get disabled for this input. Thus the data input is routed to the output Y₀.

When the selection line input, S₁S₀ = 01, the second AND gate is enabled and so the data input is directed to the output Y₁.

When S₁S₀ = 10, the third AND gate gets enabled, which will drive the data input D to the output terminal Y₂. Similarly, for S₁S₀ = 11, the AND gate at the bottom will be enabled and so the data input D will be at the output Y₃.

The truth table shown below explains the operation of 1 : 4 demultiplexer.

1 : 8 demultiplexer

Similar to the 1 to 4 demux, 1-to-8 demultiplexer performs the transfer of single data to any one of the 8 possible outputs. It has 3 selection lines to distribute the data to the output.

The operation is similar to a 1-to-4 demux. The following truth table or function table shows the operation of the 1-to-8 demultiplexer.

Cascading of Demultiplexers

Demultiplexers with more number of outputs can be designed by cascading two or more demux. Such a cascading connection is known as demulitplexer tree.

Let us consider an example here. Using two 1:4 demux, let us built 1:8 demux. While doing so, the data input D_in is common for both 1:4 demuxes. Common select lines B and C are connected to both the demuxes.

The most significant bit A is given to both demuxes, in such a way that, when A = 0, the demultiplexer at the top will be enabled. When A = 1, the demux at the bottom will be enabled.

Demultiplexer ICs

The IC 74154 and IC 74155 are the demultiplexer ICs, which perform 1-to-16 demux operation and 1-to-4 demux operations respectively.

Applications of demultiplexer

1. The demultiplexers are used along with multiplexers. So, in the communication system, the multiplexer is used for transmitting the information, whereas demux is used to retrieve the original message at the receiving end.
2. It is used in applications where serial to parallel conversion of binary data is needed.
3. In time-division multiplexing, used to route the single input to multiple output lines at the receiving end.

Difference between multiplexer and demultiplexer

Multiplexer	Demultiplexer
It is a data selector.	It is a data distributor.
It has multiple inputs and a single output.	It has a single input and multiple outputs.
It performs parallel to serial conversion.	It performs serial to parallel conversion.
In time-division multiplexing, mux is used at the transmitting end to transmit a single input data	In time-division multiplexing, demux is used at the receiving end to receive the single input data.