What are Multiplexers and Demultiplexers & their Application

Summary

In our modern world, data travels at incredible speeds, connecting continents and powering the devices in our pockets. But how does this immense volume of information travel so efficiently?

If you've ever wondered how a single fiber-optic cable can carry thousands of phone calls, TV channels, and internet data streams simultaneously, you're on the verge of understanding a fundamental concept in digital electronics.

The process is a bit like a highway system for data, where multiple lanes of traffic must merge into a single tunnel and then separate back into their respective lanes on the other side. T

his critical task of combining and separating data streams is handled by two remarkable circuits: the multiplexer and the demultiplexer.

They are the unsung traffic controllers of the digital world, ensuring that data gets where it needs to go without getting lost or jumbled. 

What is a Multiplexer (MUX)?

A multiplexer, often abbreviated as MUX, is a fundamental digital circuit that acts like a highly intelligent switch. Its primary job is to select one of several analog or digital input signals and forward the selected input into a single output line.

Imagine a DJ at a radio station with multiple audio sources—a turntable, a CD player, a computer, and a microphone. The DJ can only broadcast one audio source at a time.

The mixing console they use to choose which source goes live on air is performing the same function as a multiplexer. Because it selects one data input from many, a MUX is also commonly referred to as a "data selector."

The circuit has three main parts: multiple input lines, a single output line, and a set of control inputs known as "select lines."

Working Principle of a Multiplexer

The main component of a multiplexer is it select lines. These lines receive a binary code that acts as an address, determining which specific input line is electronically connected to the single output line.

The number of input lines a multiplexer can handle is directly related to the number of select lines it has. The relationship is defined by the formula 2n2n, where nn is the number of select lines.

For instance, a MUX with 2 select lines can manage 22=422=4 input lines. If you have 3 select lines, you can control 23=823=8 input lines, and so on. 

Let's consider a simple 4-to-1 MUX. It has four data inputs (let's call them D0, D1, D2, and D3), one output (Y), and two select lines (S1 and S0). By applying a binary signal to S1 and S0, we can choose which input gets passed to Y. 

• If we set S1=0 and S0=0 (binary 00), the MUX selects input D0. 
• If we set S1=0 and S0=1 (binary 01), it selects input D1. 
• If we set S1=1 and S0=0 (binary 10), it selects input D2. 
• If we set S1=1 and S0=1 (binary 11), it selects input D3.

In this way, the select lines act as a digital address that directs the flow of data through the circuit. 

Block Diagram of a Multiplexer

Types of Multiplexers 

Multiplexers come in various configurations based on the number of inputs they can handle. The most common types are 2-to-1, 4-to-1, 8-to-1, and 16-to-1. 

2-to-1 Multiplexer

This is the simplest form of a multiplexer. It has two input lines (D0, D1), one select line (S0), and one output line (Y). When the select line S0 is set to 0, the output Y is connected to D0. When S0 is set to 1, Y is connected to D1. 

Block Diagram: 

The block diagram shows a rectangular block labeled "2-to-1 MUX" with two input lines on one side, one output line on the opposite side, and one select line entering from the bottom or top.

Truth Table:

S0 (Select)	Output (Y)
0	D0
1	D1

 

4-to-1 Multiplexer 

This is a widely used configuration in many digital systems. A 4-to-1 MUX has four input lines (D0-D3), two select lines (S1, S0), and one output line (Y).

The two select lines can form four different binary combinations (00, 01, 10, 11), each corresponding to one of the four inputs. 

Block Diagram:

The diagram depicts a block with four inputs, one output, and two select lines.

Truth Table:

S1	S0	Output (Y)
0	0	D0
0	1	D1
1	0	D2
1	1	D3

 

8-to-1 Multiplexer 

As the number of inputs grows, so does the capability of the multiplexer. An 8-to-1 MUX has eight input lines (D0-D7), three select lines (S2, S1, S0), and one output line (Y).

The three select lines provide 23=823=8 possible binary combinations, allowing it to select any of the eight inputs. 

Block Diagram:

The block diagram shows eight inputs, one output, and three select lines.

Truth Table:

S2	S1	S0	Output (Y)
0	0	0	D0
0	0	1	D1
0	1	0	D2
0	1	1	D3
1	0	0	D4
1	0	1	D5
1	1	0	D6
1	1	1	D7

 

16-to-1 Multiplexer 

A 16-to-1 Multiplexer (MUX) is a combinational logic circuit that directs one of sixteen separate input lines to a single output line.

This device is crucial in complex digital systems where numerous data sources need to be managed and selected efficiently. 

A 16-to-1 MUX features sixteen data inputs (labeled D0 to D15), one output (Y), and four select lines (S3, S2, S1, S0). Since 24=1624=16, the four select lines can generate 16 unique binary combinations, from 0000 to 1111.

Each binary code corresponds to a specific input line, determining which one is connected to the output. For example, if the select lines are set to 1011 (decimal 11), the MUX will select input D11 and pass its signal to the output Y. 

Block Diagram:

The block diagram of a 16-to-1 MUX is represented as a rectangular block. It shows 16 input lines on one side, a single output line on the opposite side, and four select lines typically shown entering from the bottom or top of the block.

Truth Table:

The relationship between the select lines and the chosen input is defined by the following truth table:

S3	S2	S1	S0	Output (Y)
0	0	0	0	D0
0	0	0	1	D1
0	0	1	0	D2
0	0	1	1	D3
0	1	0	0	D4
0	1	0	1	D5
0	1	1	0	D6
0	1	1	1	D7
1	0	0	0	D8
1	0	0	1	D9
1	0	1	0	D10
1	0	1	1	D11
1	1	0	0	D12
1	1	0	1	D13
1	1	1	0	D14
1	1	1	1	D15

 

A 16-to-1 MUX can also be constructed by combining smaller multiplexers. For instance, two 8-to-1 multiplexers and one 2-to-1 multiplexer can be interconnected to achieve the same functionality, which simplifies design and implementation.

Applications of Multiplexer 

The ability to channel multiple data streams into one makes multiplexers indispensable in many areas of technology. 

• Communication Systems: Their primary application is in communication networks. Multiplexers are used to transmit multiple signals—such as telephone calls, television channels, or internet data—over a single transmission line, like a fiber-optic cable or copper wire. This technique, known as Time-Division Multiplexing (TDM), significantly increases the efficiency of the communication channel. 
• Computer Memory: Inside a computer, multiplexers are used to select a specific memory chip or a specific memory location from a bank of memory modules. The address bus of the computer acts as the select lines for the MUX. 
• Telephone Network: In the public telephone network, multiple phone lines from different homes and offices are connected to a central exchange. Multiplexers are used to combine these calls onto a single high-capacity line for long-distance transmission. 
• Arithmetic Logic Unit (ALU): An ALU inside a CPU performs various arithmetic and logical operations. A multiplexer is used to select the output of one of these operations and pass it on to a storage register. 

Advantages of multiplexers 

• They reduce the number of wires or communication channels required, which in turn reduces circuit cost and complexity. 
• They allow the implementation of complex logic functions using a single Integrated Circuit (IC). 
• They increase the efficiency of communication channels by allowing them to be shared among multiple signals. 
• Simplification of system design is a major benefit. 

Disadvantages of multiplexers 

• The MUX and its counterpart, the DEMUX, introduce a slight delay in the signal path, known as propagation delay. 
• The bandwidth of the single output channel must be greater than or equal to the sum of the bandwidths of the input signals. 
• Additional circuitry is needed for the select lines and their control logic. 

What is a Demultiplexer (DEMUX)? 

A demultiplexer, or DEMUX, is the exact opposite of a multiplexer. If a MUX is a data selector, a De-Multiplexer is a data distributor. It takes a single input signal and routes it to one of several output lines.

The select lines on the demultiplexer determine which output line receives the input data. Think of a single water pipe feeding a sprinkler system.

By turning different valves (the select lines), you can direct the water (the input signal) to different sprinkler heads (the output lines). 

Working Principle of a Demultiplexer 

Just like a multiplexer, a DEMUX uses select lines to perform its function. The number of output lines is determined by the number of select lines using the same 2n2n formula.

The binary code applied to the select lines determines which output line is activated to receive the data from the single input line. All other output lines remain in their default state, which is typically a logic LOW (0).

The combination of the control signal on the DEMUX input lines and the data line determines the state of the outputs. 

For example, a 1-to-4 demultiplexer has one data input (D), two select lines (S1 and S0), and four output lines (Y0, Y1, Y2, and Y3). 

• If S1=0 and S0=0, the input data D is passed to output Y0. 
• If S1=0 and S0=1, the input data D is passed to output Y1. 
• If S1=1 and S0=0, the input data D is passed to output Y2. 
• If S1=1 and S0=1, the input data D is passed to output Y3. 

Block Diagram of a Demultiplexer

Types of Demultiplexers 

Demultiplexers are categorized based on their input-to-output configuration, such as 1-to-2, 1-to-4, 1-to-8, and 1-to-16 Demultiplexer 

1-to-2 Demultiplexer 

This is the most basic form of a demultiplexer. A 1-to-2 DEMUX has one data input (D), one select line (S0), and two output lines (Y0 and Y1).

When the select line S0 is set to logic 0, the data from the input is passed to output Y0. When S0 is set to 1, the data is passed to output Y1. The unselected output remains at a logic 0 state. 

Block Diagram:

The diagram shows a block with a single input line, two output lines, and one select line controlling the output path.

Truth Table:

Select (S0)	Input (D)	Output (Y1)	Output (Y0)
0	D	0	D
1	D	D	0

 

1-to-4 Demultiplexer 

This type routes a single input to one of four possible destinations. A 1-to-4 DEMUX consists of one data input (D), two select lines (S1, S0), and four output lines (Y0 to Y3).

The two select lines can form four binary combinations (00, 01, 10, 11), and each combination directs the input data to a unique output line. For instance, a select code of 10 will activate output Y2. 

Block Diagram:

Its block diagram features a single input, four outputs, and two select lines to control the routing. 

Truth Table:

S1	S0	Output (Y3)	Output (Y2)	Output (Y1)	Output (Y0)
0	0	0	0	0	D
0	1	0	0	D	0
1	0	0	D	0	0
1	1	D	0	0	0

 

1-to-8 Demultiplexer 

This demultiplexer expands the distribution capability to eight output lines. 

This circuit has one data input (D), three select lines (S2, S1, S0), and eight outputs (Y0 to Y7). With three select lines, there are 23=823=8 possible binary combinations, from 000 to 111.

Each combination corresponds to one of the eight outputs, allowing the single input signal to be routed to any of them. 

Block Diagram:

The diagram shows a single data input being directed to one of eight outputs, controlled by three select lines. 

Truth Table:

S2	S1	S0	Y7	Y6	Y5	Y4	Y3	Y2	Y1	Y0
0	0	0	0	0	0	0	0	0	0	D
0	0	1	0	0	0	0	0	0	D	0
0	1	0	0	0	0	0	0	D	0	0
0	1	1	0	0	0	0	D	0	0	0
1	0	0	0	0	0	D	0	0	0	0
1	0	1	0	0	D	0	0	0	0	0
1	1	0	0	D	0	0	0	0	0	0
1	1	1	D	0	0	0	0	0	0	0

 

1-to-16 Demultiplexer 

This is a larger demultiplexer used in more complex data distribution systems. A 1-to-16 DEMUX has one data input (D), four select lines (S3, S2, S1, S0), and sixteen outputs (Y0 to Y15).

The four select lines provide 24=1624=16 unique binary codes, from 0000 to 1111, enabling the distribution of the input signal to any of the sixteen outputs. 

Block Diagram:

The block diagram illustrates a single input connected to one of sixteen potential outputs, with the path determined by the four select lines.

Truth Table:

S3	S2	S1	S0	Y15	Y14	Y13	Y12	Y11	Y10	Y9	Y8	Y7	Y6	Y5	Y4	Y3	Y2	Y1	Y0
0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	D
0	0	0	1	0	0	0	0	0	0	0	0	0	0	0	0	0	0	D	0
0	0	1	0	0	0	0	0	0	0	0	0	0	0	0	0	0	D	0	0
0	0	1	1	0	0	0	0	0	0	0	0	0	0	0	0	D	0	0	0
0	1	0	0	0	0	0	0	0	0	0	0	0	0	0	D	0	0	0	0
0	1	0	1	0	0	0	0	0	0	0	0	0	0	D	0	0	0	0	0
0	1	1	0	0	0	0	0	0	0	0	0	0	D	0	0	0	0	0	0
0	1	1	1	0	0	0	0	0	0	0	0	D	0	0	0	0	0	0	0
1	0	0	0	0	0	0	0	0	0	0	D	0	0	0	0	0	0	0	0
1	0	0	1	0	0	0	0	0	0	D	0	0	0	0	0	0	0	0	0
1	0	1	0	0	0	0	0	0	D	0	0	0	0	0	0	0	0	0	0
1	0	1	1	0	0	0	0	D	0	0	0	0	0	0	0	0	0	0	0
1	1	0	0	0	0	0	D	0	0	0	0	0	0	0	0	0	0	0	0
1	1	0	1	0	0	D	0	0	0	0	0	0	0	0	0	0	0	0	0
1	1	1	0	0	D	0	0	0	0	0	0	0	0	0	0	0	0	0	0
1	1	1	1	D	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0

 

Applications of Demultiplexers 

The Applications of DEMUX are often found in conjunction with multiplexers. 

• Communication Systems: At the receiving end of a communication channel where a multiplexer was used, a demultiplexer is required to separate the combined signals back into their original forms. For every MUX, there is a corresponding DEMUX to restore the data. 
• Serial-to-Parallel Converter: A demultiplexer can be used to convert a serial stream of data (bits arriving one after another) into parallel data. By cycling through the select lines in sync with the incoming data, each bit can be sent to a different output line, which can then be read simultaneously. 
• ALU (Arithmetic Logic Unit): An ALU can generate a result that needs to be stored in one of several possible registers. A demultiplexer can take the ALU's output as its data input and use control signals to route it to the correct destination register. 
• Decoder Circuits: A demultiplexer can be configured to act as a binary decoder, which is a circuit that converts a binary code into a single active output. 

Advantages of Demultiplexers 

• They allow a single data source to communicate with multiple destination devices. 
• They are essential for converting serial data back into a parallel format. 
• Using a DEMUX simplifies the design of systems that need to distribute a signal to multiple locations. 
• They are a core component in building robust communication systems. 

Disadvantages of Demultiplexers 

• Like multiplexers, they introduce a propagation delay into the circuit. 
• Only one output line is active at any given time, which can be a limitation for some designs. 
• The control logic for the select lines adds to the overall complexity and potential points of failure. 

Conclusion 

From the complex networks that power the internet to the microprocessors inside our computers, multiplexers and demultiplexers are the silent workhorses that make efficient data management possible.

By allowing us to combine multiple signals onto a single path and then separate them at the destination, they serve as the fundamental building blocks of modern digital communication and computation.

Their principles of selection and distribution help reduce physical complexity, lower costs, and enable the high-speed data transmission we rely on every day.

While they may operate behind the scenes, their role is crucial in transforming digital theory into the practical, interconnected world we inhabit.