A multiplexer (or mux) is a device that selects one of several digital input signals and directs it to a single output. A multiplexer of 2n inputs has n select lines, which are used to select which input line to send to the output. A multiplexer is also called a data selector.

The reverse of the digital multiplexer is the digital demultiplexer

4 to 1 multiplexer

A 4 to 1 multiplexer uses 2 select lines (S0, S1) to determine which one of the 4 inputs (I0 - I3) is routed to the output (Z). Its characteristics can be described in the following simplified truth table.

  • Change the select inputs S1, S0
  • Change the selected input (I0 - I3) and observe that it is routed to the output Z
  • Larger multiplexers can be constructed by chaining smaller multiplexers together. Click on the 4 to 1 MUX sub circuit to see that it is made up of 3 cascading 2 to 1 MUX.

Mux as PLD

Multiplexers (Mux) can also be used as programmable logic devices (PLD). By setting the input signals to the output of the truth table of the custom logic circuit, a programmable custom circuit is created. The selector inputs are the logic inputs of this custom circuit.

Set the 4 to 1 Mux inputs I0 - I3 according to the following truth table. Observe that the output (Z) with respect to the select lines S0, S1. You have implemented an XOR gate. Try to implement the other basic gates NOR, AND etc using the 4 to 1 Mux.

2 to 1 multiplexer

A 2 to 1 multiplexer uses 1 select line (S) to determine which one of the 2 inputs (I0, I1) is routed to the output (Z). Its simplified truth table is:

Note the full truth table that describes the 2 to 1 MUX completely

Learn by Doing

Design a 2 to 1 Multiplexer to deepen your understanding of the circuit.