Demultiplexer (DEMUX)
A 1:4 DEMUX routes a single data input (D) to one of four outputs (Y0–Y3) based on select lines S0, S1. The opposite operation of a MUX. Implemented as IC 74HC139.
1:4 DEMUX — IC 74HC139
D = 0
S = 00
D → Y0
Function Table
| S1 | S0 | Y0 | Y1 | Y2 | Y3 |
|---|---|---|---|---|---|
| 0 | 0 | D | 0 | 0 | 0 |
| 0 | 1 | 0 | D | 0 | 0 |
| 1 | 0 | 0 | 0 | D | 0 |
| 1 | 1 | 0 | 0 | 0 | D |
The selected output Yn = D. All other outputs remain 0.
How DEMUX Works
Yn = D · minterm(S1,S0)
Each output is AND-ed with its select minterm. Only the selected output mirrors D; all others are 0.
MUX vs DEMUX
MUX = many inputs → 1 output (combine signals)
DEMUX = 1 input → many outputs (distribute signal)
Also used as a Decoder
When D=1 (always enabled), a DEMUX becomes a binary decoder — converting a 2-bit select into 1-of-4 active outputs.