Multiplexer – Digital Circuits

Technical Studies – Study 29

Introduction

A multiplexer is a circuit that contains 2n inputs and n selection lines. It contains one yield or simply an output line. For 2n inputs, ‘n’ selection lines are present.

The Multiplexer can simply be called as “MUX” or “MPX” and it is a combinational logic circuit designed to switch one of several input lines through to a single common output line by using a control signal. Multiplexers act like very fast-acting multi-position rotary switches that regulate multiple input lines one at a time to the output.

Here is the block diagram of the multiplexer.

Block Diagram of multiplexer

2-input multiplexer

A 2-input multiplexer consists of two inputs D0 and D1, one select line S, and one output Y. Depending on the select line the output is connected to either of the inputs. As it contains only two input signals, the inputs can be connected to the output in only two ways.

Here is the block diagram of a 2-input multiplexer.

Block Diagram of 2-input multiplexer

We can understand the operation of a 2-input multiplexer by using two AND gates, one Inverter, and one OR gate. Notice the below circuit –

Logic Diagram of 2-input multiplexer

In the above circuit, when S is 0 the first AND gate is on and output Y is D0.

When S is 1 the second AND gate is on and output Y is D1.

So, the output of a 2-input multiplexer will be Y= D0S + D1S

Below is the truth table of a 2-input multiplexer.

Truth Table of 2-input multiplexer

4-input multiplexer

4-input multiplexer contains four data inputs D3, D2, D1 & D0, two select lines S1 & S0, and one output Y. According to the presence of two select lines, one of these four data inputs will be connected to the output.

Here is the block diagram of a 4-input multiplexer.

Truth Table of 4-input multiplexer

You can understand the function of a 4-input multiplexer by understanding the below circuit.

Logic diagram of 4-input multiplexer

In the above circuit, when S0 and S1 both are 0 output Y is D0.

When S1 is 0 and S0 is 1, output Y is D1.

When S1 is 1 and S0 is 0, output Y is D2.

When S1 and S0 both are 1, output Y is D3.

Now, the truth table of the 4-input multiplexer will be –

Truth Table of 4-input multiplexer

We can easily understand the operation and can implement the 8-input Multiplexer and 16-input multiplexer by using 4-input multiplexers.

8-input multiplexer

The 8-to-1 multiplexer consists of 8 input lines D7, D6, D5, D4, D3, D2, D1, D0, one output line Y, and 3 selection Lines S2, S1, S0. We can implement this 8 input multiplexer by using lower order multiplexers.

Here are the block diagram and truth table of an 8-input multiplexer.

Block Diagram of 8-input multiplexer
Truth Table of 8-input multiplexer

You can draw the logic diagram of an 8-input multiplexer by using the truth table given above.    

Also, the function of a 16-input multiplexer can be realized by using two 8-input and one 2-input multiplexer.

Multiplexer Applications:

Communication System

Communication systems combine both a communication network and a transmission system. A multiplexer can increase the efficiency of the communication system by allowing the transmission of data.

Computer Memory

Multiplexers can maintain a huge amount of memory in the computers and reduce the number of copper lines required to connect the memory to other parts of the computer.

Telephone Network

In telephone networks, a multiplexer can integrate multiple audio signals on a single line of transmission.

Transmission from the Computer System of a Satellite

A multiplexer can transmit the data signals from the computer system of a satellite to the ground system by using a GSM satellite. 

Multiplexer ICs

IC 74157 contains four 2-input multiplexers. IC 74153 contains two 4-input multiplexers. IC 74151A contains one 8-input multiplexer.

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