Multiplexer – Demultiplexer
Definition
A multiplexer is a combinational circuit that selects one of several input signals and transfers it to a single output line based on the values of the select lines.
A demultiplexer is a combinational circuit that takes a single input signal and routes it to one of many output lines according to the select lines.
In simple terms:
MUX
- many inputs → one output
DEMUX
- one input → many outputs
Main Content
1. Multiplexer (MUX)
- A multiplexer is often called a data selector because it selects one input from multiple input lines.
- If there are 2^n input lines, then the MUX needs n select lines to choose one of them.
How it works
A MUX uses select lines to decide which input should appear at the output.
For example, a 4-to-1 multiplexer has:
- 4 input lines: I0, I1, I2, I3
- 2 select lines: S1, S0
- 1 output line: Y
The output depends on the select combination:
| S1 | S0 | Output |
|---|---|---|
| 0 | 0 | I0 |
| 0 | 1 | I1 |
| 1 | 0 | I2 |
| 1 | 1 | I3 |
Logic expression for 4:1 MUX
For inputs I0, I1, I2, I3 and select lines S1, S0:
This expression shows that only one input term becomes active at a time based on the select lines.
Example of use
A MUX can be used to choose:
- one of several sensor signals
- one of multiple data channels
- one of many arithmetic results inside a processor
ASCII illustration
For a 4:1 MUX:
I0 ─┐
I1 ─┼──> [ 4:1 MUX ] ───> Y
I2 ─┼
I3 ─┘
S1, S0
2. Demultiplexer (DEMUX)
- A demultiplexer is often called a data distributor because it sends one input to one of several outputs.
- If there are 2^n output lines, then the DEMUX needs n select lines to choose the output line.
How it works
A DEMUX takes:
- 1 input line
- select lines to determine the destination
- multiple output lines
For a 1-to-4 demultiplexer:
- 1 input: D
- 2 select lines: S1, S0
- 4 outputs: Y0, Y1, Y2, Y3
The input goes to only one output depending on the select lines.
| S1 | S0 | Active Output |
|---|---|---|
| 0 | 0 | Y0 |
| 0 | 1 | Y1 |
| 1 | 0 | Y2 |
| 1 | 1 | Y3 |
Logic expressions for 1:4 DEMUX
If input is D, then:
Only one output receives the input at a time; all others remain 0.
Example of use
A DEMUX can be used to:
- send a data stream to one selected device
- route signals in communication networks
- drive output LEDs one by one using control signals
ASCII illustration
For a 1:4 DEMUX:
┌──> Y0
├──> Y1
D ─> [1:4 DEMUX]
├──> Y2
└──> Y3
S1, S0
3. Comparison and Relationship Between MUX and DEMUX
- A multiplexer performs the reverse function of a demultiplexer.
- A MUX combines many inputs into one line, while a DEMUX takes one line and spreads it into many outputs.
- They often work together in data communication systems: a MUX at the sending side and a DEMUX at the receiving side.
Major differences
| Feature | Multiplexer | Demultiplexer |
|---|---|---|
| Inputs | Many | One |
| Outputs | One | Many |
| Function | Selects one input | Routes input to one output |
| Also called | Data selector | Data distributor |
| Main use | Channel sharing | Signal distribution |
Example in communication
Suppose 4 sensors are connected to one microcontroller input through a MUX. The microcontroller reads them one by one. At the other side, a DEMUX can direct the processed signal to one of several output devices.
Why they are important
- They reduce the number of wires required in a circuit.
- They simplify hardware design.
- They improve efficient data transfer and switching.
Working / Process
1. Select lines are given a binary value
- The select lines determine which input or output path is active.
- For a circuit with select lines, channels can be controlled.
2. Logical gating activates only one path
- In a MUX, the chosen input is connected to the output.
- In a DEMUX, the input is connected to only one selected output.
- The other paths remain inactive.
3. Output is produced according to selection
- The final output of a MUX is the selected input signal.
- The final outputs of a DEMUX show the input only on the selected output line.
- This process is entirely combinational, meaning it depends only on present inputs and select lines, not on memory.
Example process for a 4:1 MUX
- If S1S0 = 10, the output Y = I2
- If S1S0 = 01, the output Y = I1
Example process for a 1:4 DEMUX
- If S1S0 = 11 and D = 1, then Y3 = 1 and Y0, Y1, Y2 = 0
- If D = 0, all outputs are 0 regardless of select lines
Advantages / Applications
Reduces wiring complexity
- Many signals can share fewer lines, which saves space and cost in digital circuits.
Improves data transmission efficiency
- Multiple data sources can be transmitted through a single channel using a MUX, then separated at the destination using a DEMUX.
Widely used in real systems
- Used in computer memory selection, communication systems, CPU data routing, logic circuit design, and sensor interfacing.
Summary
- Multiplexer selects one input from many and sends it to one output.
- Demultiplexer takes one input and sends it to one selected output.
- Both are combinational circuits controlled by select lines.
- Important terms to remember: MUX, DEMUX, select lines, data selector, data distributor