The action of a demultiplexer (or demux) is opposite to that of the multiplexer. As inverse to the mux , demux is a one-to-many circuit. With the use of a demultiplexer, the binary data can be bypassed to one of its many output data lines.

What is a demultiplexer?

A demultiplexer passes information (usually binary) to one of various output lines. It consists of an input line, output lines and a few select lines. The output lines are usually in power of 2. So we have demultiplexers of type -

• 1-To-2
• 1-To-4
• 1-To-8

We'll understand the concept of demultiplexing through a 1-To-4 Demux. Same concept applies to other types of demultiplexers. The diagram below illustrates the basic idea of demultiplexing.

How many select lines?

The number of select lines is the same as the power raised. So a 1-To-2 Demux will have 1 select line, 1-To-4 will have 2 select lines and 1-To-8 will have 3 select lines to be able to send information to any one output line.

Demultiplexer design

The logic circuit built using AND and OR gates is given below

The Boolean expression for this 1-to-4 Demultiplexer above with outputs A to D and data select lines a, b is given as:

F = a'b'A + a'bB + ab'C + abD.

The truth table is as follows

The important idea here is the combinations of the two selection lines. Each line can be in an off and on state (0 and 1 respectively). So the combinations for the two lines are 2 * 2 = 4 which allows for selection of any of the four output lines. Each combination points to one particular output line.

IC 74139

IC 74139 is a dual 1-To-4 demultiplexer. It has two demultiplexers. It uses the same concept and lines as described above. The input line is basically the Enable line you'll encounter in a while.

The Pin diagram is given below

Each demux has an active LOW Enable (E). When E is HIGH all outputs are forced HIGH. An active low device is a device that either outputs 0V when triggered on or that accepts 0V as input to turn on.

Each of the two demux accept two binary weighted inputs (A0, A1) and provide four mutually exclusive active LOW outputs (O0– O3).

The datasheet is available here