A demultiplexer, sometimes abbreviated dmux, is a circuit that has one
input and more than one output. It is used when a circuit wishes to send a
signal to one of many devices. This description sounds similar to the
description given for a decoder, but a decoder is used to select among many
devices while a demultiplexer is used to send a signal among many

A demultiplexer is used often enough that it has its own schematic

The truth table for a 1-to-2 demultiplexer is

Using our 1-to-2 decoder as part of the circuit, we can express this
circuit easily

This circuit can be expanded two different ways. You can increase the
number of signals that get transmitted, or you can increase the number of
inputs that get passed through. To increase the number of inputs that get
passed through just requires a larger line decoder. Increasing the number of
signals that get transmitted is even easier.

As an example, a device that passes one set of two signals among four
signals is a “two-bit 1-to-2 demultiplexer”. Its circuit is

or by expressing the circuit as

shows that it could be two one-bit 1-to-2 demultiplexers without changing
its expected behavior.

A 1-to-4 demultiplexer can easily be built from 1-to-2 demultiplexers as

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