Multiplexers in Digital Electronics

A digital logic circuit that accepts several data inputs and allows only one of them at a time to flow through the output is called a multiplexer or MUX. This article is meant for explaining multiplexer in digital electronics, its block diagram, function, and different types. So, let us start with the basic introduction of multiplexer

What is a Multiplexer?

As already mentioned, a multiplexer, also referred to as MUX, is a combination logic circuit that is designed to accept multiple input signals and transfer only one of them through the output line. In simple words, a multiplexer is a digital logic device that selects one-out-of-N (N = 2ⁿ) input data sources and transmits the selected data to a single output line.

The multiplexer is also called data selector as it selects one from several. The block diagram of a typical 2ⁿ:1 multiplexer is shown in Figure 1.

In the case of multiplexer, the selection of desired data input to flow through the output line is controlled with the help of SELECT lines. In the block diagram of mux in Figure 1, I₀, I₁,… I_n-1, i.e., (2ⁿ) are the input lines, and "n" be the select lines. These select lines will determine which input is to be routed to the output.

Hence, the multiplexer works as a multi-position switch whose operation is controlled by digital signals. These digital control signals are applied to the select lines to determine which data input will be switched to the output line.

Function of Multiplexer

Multiplexer is a digital logic device which is used to perform multiplexing of data. Where, multiplexing simply means sharing of data. Technically, when a particular data is selected from multiple input data sources and transmitted the selected data to a single output channel, it is called multiplexing.

There are two types of multiplexing namely, frequency multiplexing and time multiplexing.

When multiple devices are connected to a single transmission line in a system. At any point of time, only one device is using the line to transmit data, then this is called time multiplexing. On the other hand, when multiple devices share a common line to transmit data but at different frequencies, it is called frequency multiplexing.

Types of Multiplexers

Based on input data lines and select lines, the multiplexer can be of several types. But, in this article, we will discuss only the following three types of multiplexers −

• 2:1 Multiplexer

• 4:1 Multiplexer

• 8:1 Multiplexer

Let us discuss each of these three multiplexers individually.

2:1 Multiplexer

The block diagram of a 2:1 multiplexer is shown in Figure 2. The 2:1 multiplexer is basic two input multiplexer which has two data input lines designated as I₀ and I₁, one data select line denoted by S and one output line denoted by Y. The 2:1 mux is used to connect two 1-bit data sources to a common designation.

In the 2:1 multiplexer, the logic level of the digital signal applied to the select line S determines which data input will pass through the output line. The operation of the 2:1 multiplexer can be understood from the following truth table.

Select Line (S)	Output (Y)
0	I0
1	I1

4:1 Multiplexer

The 4:1 multiplexer is one which has 4 data input lines, 2 select lines, and 1 output line. The block diagram of a 4:1 mux is shown in Figure 3.

Here, I₀, I₁, I₂, and I₃ are the four data input lines, S₀ and S₁ are the two select lines, and Y is the output line. Thus, the logic level of the digital signal applied to S₀ and S₁ select lines will determine which input will pass to the output line.

The operation of the 4:1 multiplexer can be understood with the help of its truth table given below.

Select Lines		Output
S1	S0	Y
0	0	I0
0	1	I1
1	0	I2
1	1	I3

8:1 Multiplexer

The block diagram of the 8:1 multiplexer is shown in Figure 4. In the 8:1 mux, there are 8 input data lines from I₀ to I₇, 3 select lines S₀, S₁, and S₂, and 1 output line Y.

Here, the three select lines, i.e., S₀, S₁, and S₂ determine which input data will pass through the output line. The following truth table helps to understand the operation of the 8:1 multiplexer.

Select Lines			Output
S2	S1	S0	Y
0	0	0	I0
0	0	1	I1
0	1	0	I2
0	1	1	I3
1	0	0	I4
1	0	1	I5
1	1	0	I6
1	1	1	I7

Applications of Multiplexers

In digital electronics, multiplexers have numerous applications in almost all types of digital systems. Some important applications of multiplexers are as follows −

• Data routing and data selection

• Parallel to series conversion

• Logic function implementation

• Generation of waveform, etc.

Conclusion

In this tutorial, we discussed in detail the different types of multiplexers used in digital electronics along with their functions and applications.