Demultiplexers

Demultiplexers

• A demultiplexer is a circuit that receives information on a single line and transmits this information on one of 2n possible output lines. The selection of specific output line is controlled by the values of n selection lines.

• The Fig. 3.18.1 shows the block diagram of a demultiplexer. It has one input data line, 2ⁿ output lines, n select lines and one enable input.

Differentiate between Multiplexer and Demultiplexer

1. Types of Demultiplexers

a. 1 : 4 Demultiplexer

• Fig. 3.18.2 shows 1 : 4 demultiplexer. The single input variable has a path to all four outputs, but the input information is directed to only one of the output lines depending on the select inputs. Enable input should be high to enable demultiplexer.

b. 1 : 8 Demultiplexer

The Fig. 3.18.3 shows 1 : 8 demultiplexer. The single input data Din has a path to all eight outputs, but the input information is directed to only one of the output lines depending on the select inputs.

2. Expanding Demultiplexers

• To provide larger output needs we can cascade two or more demultiplexer to get demultiplexer with more number of output lines. Such a connection is known as demultiplexer tree.

Ex. 3.18.1 Design 1 : 8 demultiplexer using two 1 : 4 demultiplexers.

Sol. :

Step 1 : Connect D_in signal to D_in input of both the demultiplexers.

Step 2 : Connect select lines B and C to select lines S₁ and S₀ of the both demultiplexers, respectively.

Step 3 : Connect most significant select line (A) such that when A = 0 DEMUX 1 is enabled and when A = 1 DEMUX 2 is enabled.

Ex. 3.18.2 Implement 1 : 16 demultiplexer using 1 : 4 demultiplexers.

Sol. : The 1 : 16 demultiplexer has 16 outputs. To select one of the 16 output, the circuit needs 4 (v 24 = 16) select lines. Each 1 : 4 demultiplexer requires 2 select lines.

Step 1 : Connect two least significant select lines (S₁, S₀) to select lines of four 4 : 1 demultiplexer. 

Step 2 : Connect one more 4 : 1 demultiplexer such that its four outputs are routed to the data inputs of the four demultiplexers. Connect higher select lines (S₃, S₂) to the select lines of this demultiplexer.

Examples for Practice

Ex. 3.18.3 Draw 1 : 64 demultiplexer tree using 1 : 16 demultiplexer.

Ex. 3.18.4 Draw 1 : 64 demultiplexer tree using 1 : 8 demultiplexer.

3. Implementation of Combinational Logic using Demultiplexer

Ex. 3.18.5 Implement full subtractor using demultiplexer.

AU : May-12, Marks 8

Sol. :

Step 1 : Write the truth table of full subtractor.

Step 2 : Represent output of full-subtractors in minterm form.

For full subtractor difference D function can be written as D = f (A, B, C) = ∑ m (1, 2, 4, 7) and B_out function can be written as,

B_out = F (A, B, C) = ∑ m (1, 2, 3, 7)

Step 3 : Logically OR the outputs corresponding to minterms.

With D_in input 1, demultiplexer gives minterms at the output so by logically ORing required minterms we can implement Boolean functions for full subtractor. Fig. 3.18.6 shows the implementation of full subtractor using demultiplexer.

Ex. 3.18.6 Implement the following functions using demultiplexer :

f₁ (A, B, C) = ∑ m (0, 3, 7)

f₂ (A, B, C) = ∑ m (1, 2, 5)

Sol. :

f₁ (A, B, C) = ∑ m (0, 3, 7)

f₂ (A, B, C) = ∑ m (1, 2, 5)

Implementation using 1 : 8 demultiplexer.

Examples for Practice

Ex. 3.18.7 Implement full adder using demultiplexer.

Ex. 3.18.8 Implement the following functions using demultiplexer

f₁ (A, B, C) = ∑ m (1, 5, 7)

f₂ (A, B, C) = ∑ m (3, 6, 7)

4. Applications of Demultiplexer 

1. It can be used as a decoder.

2. It can be used as a data distributer.

3. It is used in time division multiplexing at the receiving end as a data separator.

4. It can be used to implement Boolean expressions.

5. Demultiplexer ICs

Review Questions

1. Define demultiplexer.

2. Differentiate between multiplexer and demultiplexer.

3. State the applications of demultiplexer.