Digital Electronics - Demultiplexers

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What is a Demultiplexer?

A Demultiplexer is a combinational logic circuit that accepts a single input and distributes it over several output lines. Demultiplexer is also termed as DEMUX in short. As Demultiplexer is used to transmit the same data to different destinations, hence it is also known as data distributor.

There is another combinational logic circuit named multiplexer which performs opposite operation of the Demultiplexer, i.e. accepts several inputs and transmits one of them at time to the output line.

From the definition, we can state that a Demultiplexer is a 1-to-2ⁿ device. The functional block diagram of a typical 1×2ⁿ Demultiplexer is shown in Figure-1.

It can be seen that the Demultiplexer has only one data input line, 2ⁿ output lines, and n select lines. The logic level applied to select lines of the Demultiplexer determines the output channel to which the input data will be transmitted.

Demultiplexer circuit are the combinational logic circuit widely used in digital decoders and Boolean function generator circuits.

Types of Demultiplexer

Based on the number of output lines (2ⁿ), Demultiplexers can be classified into several types. Some commonly used types of Demultiplexers are −

• 1×2 Demultiplexer
• 1×4 Demultiplexer

Now, let us briefly discuss each type of Demultiplexer.

1×2 Demultiplexer

The functional block diagram of a 1×2 Demultiplexer is shown in Figure-2.

The 1×2 Demultiplexer consists of 1 input line (I), 1 select line (S), and 2 output lines (Y₀ and Y₁). The logic level applied at the select line determines the output line to which the input data will be transmitted.

The operation of the 1×2 Demultiplexer can be analyzed with the help of its function table given below.

From this function table of 1×2 Demultiplexer, we can directly derive the Boolean expression for each output as follow.

$$\mathrm{Y_{0} \: = \: \bar{S} \: I}$$

And,

$$\mathrm{Y_{1} \: = \: S \: I}$$

1×4 Demultiplexer

The functional block diagram of 1×4 Demultiplexer is shown in Figure-3.

The 1×4 Demultiplexer has 1 input line (I), 2 select line (S₀ and S₁), and 4 output lines (Y₀, Y₁, Y₂, and Y₃). The logic level applied to the select lines determines the output line to which the input data (I) will be transmitted.

The operation of the 1×4 Demultiplexer can be understood with the help of its function table given below.

From this truth table of 1×4 Demultiplexer, we can directly write the Boolean expression for each output as follow.

$$\mathrm{Y_{0} \: = \: \bar{S_{1}} \: \bar{S_{0}} \: I}$$

$$\mathrm{Y_{1} \: = \: \bar{S_{1}} \: S_{0} \: I}$$

$$\mathrm{Y_{2} \: = \: S_{1} \: \bar{S_{0}} \: I}$$

$$\mathrm{Y_{3} \: = \: S_{1} \: S_{0} \: I}$$

We can easily understand the operation of the above circuit. Similarly, you can implement 1×8 Demultiplexer and 1×16 Demultiplexer by following the same procedure.

Implementation of Higher-order Demultiplexer

Now, let us implement the following two higher-order Demultiplexers using lower-order Demultiplexers.

• 1×8 Demultiplexer
• 1×16 Demultiplexer

1×8 Deultiplexer

In this section, let us implement 1×8 Demultiplexer using 1×4 Demultiplexers and 1×2 Demultiplexer. We know that 1×4 Demultiplexer has single input, two selection lines and four outputs. Whereas, 1×8 Demultiplexer has single input, three selection lines and eight outputs.

So, we require two 1×4 Demultiplexers in second stage in order to get the final eight outputs. Since, the number of inputs in second stage is two, we require 1×2 Demultiplexer in first stage so that the outputs of first stage will be the inputs of second stage. Input of this 1×2 Demultiplexer will be the overall input of 1×8 Demultiplexer.

Let the 1×8 Demultiplexer has one input I, three selection lines s₂, s₁ & s₀ and outputs Y₇ to Y₀. The Truth table of 1×8 Demultiplexer is shown below.

We can implement 1×8 Demultiplexer using lower order Multiplexers easily by considering the above Truth table. The block diagram of 1×8 Demultiplexer is shown in the following figure.

The common selection lines, s₁ & s₀ are applied to both 1×4 Demultiplexers. The outputs of upper 1×4 Demultiplexer are Y₇ to Y₄ and the outputs of lower 1×4 Demultiplexer are Y₃ to Y₀.

The other selection line, s₂ is applied to 1×2 Demultiplexer. If s₂ is zero, then one of the four outputs of lower 1×4 Demultiplexer will be equal to input, I based on the values of selection lines s₁ & s₀. Similarly, if s₂ is one, then one of the four outputs of upper 1×4 Demultiplexer will be equal to input, I based on the values of selection lines s₁ & s₀.

1×16 Demultiplexer

In this section, let us implement 1×16 Demultiplexer using 1×8 Demultiplexers and 1×2 Demultiplexer. We know that 1×8 Demultiplexer has single input, three selection lines and eight outputs. Whereas, 1×16 Demultiplexer has single input, four selection lines and sixteen outputs.

So, we require two 1×8 Demultiplexers in second stage in order to get the final sixteen outputs. Since, the number of inputs in second stage is two, we require 1×2 Demultiplexer in first stage so that the outputs of first stage will be the inputs of second stage. Input of this 1×2 Demultiplexer will be the overall input of 1×16 Demultiplexer.

Let the 1×16 Demultiplexer has one input I, four selection lines s₃, s₂, s₁ & s₀ and outputs Y₁₅ to Y₀. The block diagram of 1×16 Demultiplexer using lower order Multiplexers is shown in the following figure.

The common selection lines s₂, s₁ & s₀ are applied to both 1×8 Demultiplexers. The outputs of upper 1×8 Demultiplexer are Y₁₅ to Y₈ and the outputs of lower 1×8 Demultiplexer are Y₇ to Y₀.

The other selection line, s₃ is applied to 1×2 Demultiplexer. If s₃ is zero, then one of the eight outputs of lower 1×8 Demultiplexer will be equal to input, I based on the values of selection lines s₂, s₁ & s₀. Similarly, if s3 is one, then one of the 8 outputs of upper 1×8 Demultiplexer will be equal to input, I based on the values of selection lines s₂, s₁ & s₀.

Integrated Circuits (ICs) Working as Demultiplexer

Demultiplexer can also be built in the form of ICs. There are several types of ICs available that work as Demultiplexer. Some common of them are listed below −

• 74139 IC works as a 1×4 Demultiplexer
• 74237 IC works as a 1×8 Demultiplexer
• 74154 IC works as a 1×16 Demultiplexer

Advantages of Demultiplexer

The important advantages of Demultiplexer are given below −

• By using Demultiplexers, we can increase the efficiency of the communication systems.
• Demultiplexer can separate different signals from a mixed signal stream.
• Demultiplexer can decode the signals produced by a multiplexer.

Disadvantages of Demultiplexer

The major disadvantages of Demultiplexers are listed below −

• The use of Demultiplexer can cause wastage of bandwidth.
• The synchronization of signals can create a delay in the system.

Applications of Demultiplexer

Demultiplexer is a crucial combinational logic circuit which is used in a number of applications. Some important uses of Demultiplexers are listed below −

• Demultiplexer are used in several input and output devices for data routing.
• Demultiplexer are used in digital control systems to select one signal from a mutual stream of signals.
• Demultiplexer are also employed for data transmission in synchronous systems.
• Demultiplexer are also utilized in data acquisition systems.
• Demultiplexer can be used for generating Boolean functions.
• Demultiplexer can be used in serial to parallel converters.
• Demultiplexer are used for broadcasting of ATM packets.
• Demultiplexer can also be used to design automatic test equipment, etc.

This is all about Demultiplexer, its types, and applications.