Side-by-side comparison
| Parameter | Multiplexer | Demultiplexer |
|---|---|---|
| Function | Many inputs → one output (many-to-one) | One input → many outputs (one-to-many) |
| Select Lines for N channels | log2(N) select lines | log2(N) select lines |
| Data Lines | 2^n inputs, 1 output | 1 input, 2^n outputs |
| Typical IC (4:1) | 74HC153 (dual 4:1) | 74HC139 (dual 1:4) |
| Typical IC (8:1 / 1:8) | 74HC151 | 74HC138 |
| Enable Pin | Active-low strobe on 74HC151 | Active-low G1, G2A, G2B on 74HC138 |
| Use as Decoder | Can implement Boolean functions | 74HC138 directly used as 3:8 decoder |
| Cascading | Use enable to cascade to 16:1 | Use enable to cascade to 1:16 |
| Power (74HC, 5V) | ~80 µA quiescent | ~80 µA quiescent |
| Application | Function generator, data selector, bus sharing | Memory chip select, address decoding, data routing |
Key differences
A MUX routes one of 2^n inputs to a single output using n select lines — the 74HC151 selects 1 of 8 data lines. A DEMUX does the inverse: routes one input to one of 2^n outputs, selected by n lines. The 74HC138 doubles as both a 1:8 DEMUX and a 3:8 decoder since the active-low output enables exactly one line at a time. Critically, a 2:1 MUX can implement any 2-variable Boolean function directly, making it a programmable logic element — a fact examiners love to test.
When to use Multiplexer
Use a MUX (74HC151) when multiple data sources share a single transmission line, such as combining eight sensor readings onto a single ADC input by time-division switching at 1 kHz.
When to use Demultiplexer
Use a DEMUX (74HC138) when a single address or data bus must select one of several peripheral chips, such as memory address decoding in an 8051 system where A15-A13 select one of eight 8 KB ROM banks.
Recommendation
For exam problems, recognise that the 74HC138 is the most versatile DEMUX/decoder IC — choose it when any address decoding or output selection is needed. For data selection and Boolean function implementation, choose the 74HC151 MUX.
Exam tip: Examiners test MUX-based combinational logic implementation — be able to realise a 3-variable Boolean function using a 4:1 MUX by assigning one variable to the data inputs and two to select lines.
Interview tip: Interviewers expect you to explain why the 74HC138 can be used as a minterm generator and how cascading two 74HC138s with a single extra gate gives you a 4:16 decoder.