Demultiplexers

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 devices.

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

dmux schematic symbol

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

dmux truth table

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

dmux circuit

dmux ladder

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

Expanded dmux

or by expressing the circuit as

Expanded dmux simplified circuit

Expanded dmux simplified ladder

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 follows.

2-to-4 demultiplexer