A multiplexer funnels many signals down to one path. A demultiplexer takes that one path and spreads it back out again, exact mirror images of each other.
Both circuits are built entirely from select lines choosing between possibilities, and the same binary math decides how many lines each one needs.
Confusing the two is easy on paper, since both show up in the same data paths, often right next to each other in a design.
This guide separates the two clearly, then hands over a calculator for the one question every design runs into: how many select lines does a given input or output count actually require.
Multiplexer vs demultiplexer comes down to direction: a multiplexer is a many to one circuit that selects one of several inputs to route to a single output, while a demultiplexer is a one to many circuit that routes a single input to one of several outputs, with both using the same binary select line logic to make that choice.
A multiplexer and a demultiplexer are often described together because they solve complementary halves of the same problem, getting several signals onto one shared path and then getting them back off it.
Both circuits rely on select lines, and the number of select lines needed follows the exact same binary rule in either direction.
This article separates the two operations clearly, works through their truth tables, and includes a calculator for sizing select lines on a real design.
What a Multiplexer Actually Does
A multiplexer, often shortened to MUX, is a combinational circuit that selects one of several input signals and routes it through to a single output line.
Select lines control which input gets chosen at any given moment, acting like a rotary switch controlled by binary code instead of a hand.
A multiplexer is also called a data selector for exactly this reason, since selecting one line out of many is its entire job.
What a Demultiplexer Actually Does
A demultiplexer, shortened to DEMUX, is the reverse circuit. It takes a single input signal and routes it to exactly one of several output lines.
The same select lines that chose an input on a multiplexer now choose a destination output on a demultiplexer, same binary logic, opposite direction of travel.
A demultiplexer is also called a data distributor, since spreading one signal out across many destinations is its entire job.
MUX and DEMUX Side by Side
Multiplexer
Many inputs, one output. Select lines choose which input passes through. Also called a data selector.
Demultiplexer
One input, many outputs. Select lines choose which output receives it. Also called a data distributor.
4:1 Multiplexer and 1:4 Demultiplexer Truth Tables
| Select A | Select B | 4:1 MUX Output | 1:4 DEMUX Active Output |
|---|---|---|---|
| 0 | 0 | I0 | O0 |
| 0 | 1 | I1 | O1 |
| 1 | 0 | I2 | O2 |
| 1 | 1 | I3 | O3 |
The exact same two select lines drive both circuits. On the multiplexer they choose which input reaches the single output. On the demultiplexer they choose which single output receives the input.
How Many Select Lines Does a Design Actually Need
Every additional select line doubles how many inputs a multiplexer can choose from, or how many outputs a demultiplexer can reach, since each line adds one more binary digit to the selection code.
1 Select Line
Chooses between 2 inputs or outputs, the smallest possible MUX or DEMUX.
2 Select Lines
Chooses between 4 inputs or outputs, the classic 4:1 or 1:4 configuration.
3 Select Lines
Chooses between 8 inputs or outputs, common in 8 channel data routing.
4 Select Lines
Chooses between 16 inputs or outputs, used where a design needs a wider bus.
MUX / DEMUX Select Line Calculator
Enter how many inputs a multiplexer needs to choose from, or how many outputs a demultiplexer needs to reach, and the calculator returns the minimum select lines required.
Two Select Line Calculations Worked Through
A design needing to multiplex exactly 8 sensor inputs down to one ADC channel lands on a clean power of two, so 3 select lines exactly cover all 8 inputs with nothing wasted.
Select lines = ceil(log2(8)) = 3
Capacity = 8 inputs, 0 unused
A design needing to demultiplex a signal out to 10 separate output devices does not land on a power of two, so it needs 4 select lines, which actually provide capacity for 16 outputs, leaving 6 unused.
Select lines = ceil(log2(10)) = 4
Capacity = 16 outputs, 6 unused
Where MUX and DEMUX Pairs Show Up in Real Systems
Time division multiplexing pairs a multiplexer and demultiplexer across a communication link, letting several parallel signals share one serial line, then splitting them back apart at the far end.
Memory address decoding uses a demultiplexer to route a CPU's read or write signal to exactly one memory chip out of many sharing the same data bus.
Display driving circuits often multiplex several LED or LCD segments onto shared drive lines, cycling through them fast enough that the eye sees a steady image instead of a flicker.
Multiplexer and Demultiplexer Do's and Don'ts
✓ Do
- Round select lines up to the next whole number, a fractional select line does not exist
- Check unused capacity before committing to a select line count, unused lines waste board space
- Keep the select code stable while switching, glitches on select lines cause momentary wrong routing
- Use a demultiplexer, not a multiplexer, whenever a single source must reach many destinations
✗ Don't
- Confuse the two directions, a multiplexer never widens a signal out to many outputs
- Assume every input count lands on a clean power of two, most real designs don't
- Forget that both circuits are purely combinational, with no memory of a prior selection
- Overlook propagation delay through the select logic on very wide, many line designs
Resources on Multiplexers and Demultiplexers
Multiplexer vs Demultiplexer Questions Students Ask
External References
- GeeksforGeeks: Difference Between Multiplexer and Demultiplexer
- Electronics Tutorials: The Multiplexer (MUX) and Multiplexing Tutorial
What We Learn Today
- A multiplexer selects one of several inputs to a single output. A demultiplexer routes a single input to one of several outputs, the exact mirror operation.
- Both use select lines governed by the same rule: n select lines handle 2 to the power of n inputs or outputs.
- When an input or output count isn't a clean power of two, the select line count rounds up, leaving some capacity unused.
- MUX and DEMUX pairs commonly work together, sharing a link then splitting it back apart, as in time division multiplexing.