Introduction to Multiplexer 17

[otw_is sidebar=otw-sidebar-1]Digital systems have become so much successful that they are found in almost all areas of life that is telecommunication, space exploration, entertainment, medical equipment and many more. The digital design techniques and technologies have changed remarkably over past forty years. Huge of network of gates that were once employed in order to accomplish various tasks are now replaced with microprocessor and microcontroller. Field Programmable Gate Arrays (FPGAs) and Application Specific Integrated Circuits (ASICs) have made prototyping of digital circuits a lot easier and the time for prototype development have also reduced significantly.

Introduction to Multiplexer

Some of the basic digital circuits are still in use in huge digital systems. These digital circuits implements a particular logic determined by their Boolean Expression and their unique design among them multiplexer is an inevitable digital logic circuit.

What is Multiplexer?

In digital electronics the Multiplexer also known as the data selector or MUX is the multiple input and single output digital circuit. The purpose of the Multiplexer is to select one of the several input signals and forward it at the output. N number of input signals are simultaneously connected to the Multiplexer and a specific input signal that is determined by the “select input” of the multiplexer is forwarded at the output. A multiplexer can either be analog or digital, an analog multiplexer is designed to select the analog signals at its input and digital multiplexer is designed to select the digital signals at its input and deliver one of them at the output. The schematic symbol and working principle of the Multiplexer is as represented in the following figure:

The above figure represents a 2 to 1 multiplexer that is it has 2 inputs A and B and one output Z and select pin denoted by S determine which input connects to the output. The operation of the multiplexer can be understood with the help of the second figure; when select pin is at logic high (1) input A will be connected to the output and when select pin is at logic low (0) the input B is connected to the output.

Boolean Expression and Truth Table of the Multiplexer:

Each digital circuit operates on its input in a particular manner to generate the output. The manner in which the digital circuit responds to its input is referred to as the logic of the digital circuit. The logic of the digital circuit is determined by the Boolean Expression which relates the input and output of the digital circuit in the form of the equation. It is important to note here that the same logic can be implemented with different Boolean Expressions. In a similar way the truth table of the digital circuit represents the relationship between the input and output of the digital circuit in the form of the table. The Boolean Expression of the Multiplexer and its truth table is as shown in the following figure:

The above figure represents the truth table and Boolean Expression of 2 to 1 multiplexer; notice from the table that for select input S 0, output Z is equal to the input A and for select input 1 the output is equal input B. Also note that only one select input is required to switch between two input signals. With two select pins up to four input signals can be selected, with three select pin up to eight input signals can be selected, with 4 select pins up to sixteen input signals can be selected and so on.

The schematic symbols of 4 to 1 multiplexer, 8 to 1 multiplexer and 16 to 1 multiplexer are shown in the following figure. The 4 to 1 multiplexer has two select pins as four different combinations are possible with two inputs and each input signal is selected corresponding to a specific combination. Similarly 16 to 1 multiplexer has four select pins as 16 different combinations are possible.

Applications of Multiplexer:

The applications of Multiplexer are diverse but its most common application is there where there is a need of time division multiplexing of a number of signals. Multiplexers are used in situations when data from different sources are to be fed to the single input. Multiplexers along with its counterpart De-multiplexers (DEMUX) is used to cater the issues related to the space constraints. In some situations the board size is quite small and there is a need to connect multiple inputs to the multiple outputs, thus instead routing copper trace for each individual signal all of the input signals are connected to the MUX and the output of the MUX is connected to the input of the DEMUX. Thus the intended input signal can be connected to the respective output without routing different copper traces. In addition to catering the space constraints this technique also helps to manage the cost issues. Multiplexers can also be found in Source Programmable Counters.

Multiplexer IC 74hc157:

The multiplexer ICs are common products and are readily available in the market. 74157 is a popular Quad 2 to 1 multiplexer that this IC has four 2 to 1 multiplexers all integrated on the same chip. The following figure shows the pinout and working diagram of the multiplexer IC.

All of the multiplexers have single select input that is whether input signal at A1, A2, A3, A4 or B1, B2, B3, B4 are forwarded at Y1, Y2, Y3, Y4 respectively is determined by single select pin.

That is all for now I hope this article would be helpful for you. In the previous article I have discussed about the full adder and full subtractor. In the next article I will come up with more interesting engineering topics. Till then stay connected, keep reading and enjoy learning.