Question

What is a multiplexer (MUX) and how does it select inputs?

Answer 1

A multiplexer, commonly known as a MUX, is a fundamental digital logic device used in electronics and digital systems. Its primary function is to select one of several input signals and forward it to a single output line based on the control inputs. In other words, a MUX allows you to choose between multiple inputs and pass the selected input to the output.

The basic building block of a multiplexer is the 2-to-1 MUX, which has two input lines (A and B), one output line (Y), and a control input (often denoted as S or SEL) that determines which input is chosen. The MUX selects either input A or input B based on the value of the control input.

Here's the truth table for a 2-to-1 MUX:

SEL    Input A    Input B    Output Y
0    0    0    0
0    0    1    0
1    1    0    0
1    1    1    1

When the control input (SEL) is 0, the MUX selects input A and passes it to the output Y. When the control input is 1, the MUX selects input B and forwards it to the output Y.

For higher-order multiplexers, such as 4-to-1, 8-to-1, or more, the number of input lines and control inputs increases accordingly. For example, a 4-to-1 MUX has four input lines (A, B, C, and D), one output line (Y), and two control inputs (S1 and S0) to select among the four inputs. The number of control inputs determines the number of input lines the MUX can handle, following the pattern of 2^n inputs, where n is the number of control inputs.

The general formula for the number of inputs (n) of a multiplexer is 2^n, and the number of control inputs required is n.

To summarize, a multiplexer is a versatile digital logic component used for data selection by choosing one input among several based on the value of control inputs. It plays a vital role in data routing, memory addressing, and many other digital circuit applications.