Multiplexers and Demultiplexers in Digital Circuits  

Multiplexers and demultiplexers are devices that route signals. A multiplexer also called a data selector takes one of several inputs and connects it to a single output. The input line that is connected is determined by the binary applied to the address lines. The diagram below shows a multiplexer with four input lines:

The two address lines can express numbers zero to three corresponding to the four inputs. For instance, if A₁A₀ = 10, the logic level at D₂ appears at the output pin Y. The number of inputs on a multiplexer on a multiplexer is not limited to four and is related to the number of address lines. If there are n selection lines, then the number of maximum possible input lines is 2ⁿ and the multiplexer is referred to as a 2ⁿ-to-1 multiplexer or 2ⁿ x 1 multiplexer.

A digital multiplexer is unidirectional: the digital level at the selected data line is transferred to the output Y, but a digital level applied at Y will not be transferred to the selected data line. Lastly, most multiplexers have an Enable input or Strobe input as shown in figure (a) above. If this input is low, the multiplexer is enabled and the above described operation occurs. If Enable is high, the output is not connected to any data line and has high impedance to ground. This permits the disconnection of Y from any device that follows it. The small circle on the Enable input indicate the fact the logic is reversed (a high level would normally be associated with turning a device ON, not OFF).

In terms of ICs, there are multiplexers of various input line capacities. For example, the 74HC151 8-line-to-1-line multiplexer uses three select inputs (S₀, S₁, S₂) to select among one of eight possible data inputs (I₀ to I₇) to be routed to the output.

To create a larger multiplexer, you can combine two smaller multiplexers together. For example, the figure (c) below shows two 8-line-to-1-line 74HC151 ICs combined to create a 16-line-to-1 line multiplexer. Alternatively you could just use the 16-line-1-line multiplexer IC like the 74HC150 shown in figure (d).

A demultiplexers or data distributor is the opposite of a multiplexer. It takes a single data input and routes it to one of several possible outputs. Let’s consider the demultiplexer illustrated below:

In this case, the logic level at the single input D is routed to one of several output lines Y in accordance with the binary number applied to the address lines. Unused output lines have high impedance to ground, as does the selected output if the Enable input is high. As with the multiplexers, digital demultiplexers are unidirectional. An IC that contains two functionally separate four-line demultiplexers is the 74HC139 shown in figure (f) below.

You can also read: The Basics of Ladder Diagrams for Programming PLCs