Multiplexers

A multiplexer (MUX for short) is a logic circuit that selects one input out of many to pass on through to the output. An example would be a car stereo choosing to play music from the radio, a CD, or bluetooth connection. Below is a very minimal example of a two-input multiplexer.

The select input \(S\) determines which of the inputs A or B get passed on. When \(S\) is High, it will feed a 1 into the top AND gate and a 0 into the bottom AND. This means that the bootom AND will always output 0 (because \(\bar{S}\) is 0), and the top AND will output whatever the value of \(A\) is. When these are ORed together the final ouput will also be whatever \(A\) is.

This same structure can be expanded for more inputs. For example, a 4-input multiplexer would have the select control be a 2-bit binary number \(S_1S_0\). Each AND gate would then have three inputs.

MUX are often used to convert parallel data into serial data by having the select inputs cycle regularly. In a circuit diagram, they can be shown by a trapezoid symbol.