Demultiplexer (DEMUX)

Also called: data distributor, distributor, 1-to-4 demultiplexer, 1-to-4 DEMUX, demultiplexor

A combinational circuit that sends one data input to one of 2ⁿ outputs, chosen by n select lines. Every other output stays 0.

A demultiplexer is the mirror image of a multiplexer. A MUX picks one of many inputs for a single output. A DEMUX takes a single input and delivers it to one of many outputs.

It has one data input D, n select lines and 2ⁿ outputs. The selected output copies D. Every other output is 0.

Each output is D ANDed with the minterm of its select code. For a 1-to-4 DEMUX (S1 is the MSB):

Y0 =
Y1 =
Y2 =
Y3 =

Compare that with a decoder with an enable input: its outputs are E times each minterm. They are the same circuit. Wire the data to the enable and use the decoder's inputs as selects, and a decoder is a DEMUX. That's why chip catalogs often sell one part as a "decoder/demultiplexer".

Typical use: sharing one wire between several destinations, for example sending a stream of bits to one of several devices, one at a time.

Worked examples

Example

Routing a 1

A 1-to-4 DEMUX has D = 1 and S1 S0 = 11. Find Y3 Y2 Y1 Y0.

1.
11₂ = 3, so D is routed to Y3.
2.
Y3 = = 1 · 1 · 1 = 1.
3.
Y0, Y1 and Y2 each contain or , which is 0, so they are 0.
4.
Outputs Y3 Y2 Y1 Y0 = 1000.

Example

When the data is 0

Same DEMUX, same select 11, but now D = 0.

1.
Y3 = = 0 · 1 · 1 = 0.
2.
The other outputs are 0 as before.
3.
All four outputs are 0. The selected output still "follows D"; D just happens to be 0.

Common mistakes

Expecting the selected output to be 1 whatever D is. It copies D, so with D = 0 every output is 0.
Thinking unselected outputs keep their old values. A DEMUX is combinational: unselected outputs are 0.
Treating a DEMUX and a decoder with enable as different circuits. They are the same gates with different labels.

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