Multiplexer

A multiplexer is a combinational circuit that gates one out of its several inputs to a single output. As it selects one out of many inputs, it is also called a “data selector”. The input selected for connection to the output is controlled by a set of SELECT inputs. A typical 4-input multiplexer is illustrated in the figure below.

Figure: Schematic of a 4-input multiplexer

S0 and S1 are select inputs. Together S0 and S1 determine the input, among the Data Inputs, DI0, DI1, DI2, and DI3, that gets connected to the output Y.

The output of the multiplexer is given by:

Notice that the relationship between the SELECT inputs and the DATA inputs is G dependency.

The main parameters of concern to us are:

• Number of inputs
• Nature of outputs
• Propagation delay

The choice on the number of inputs enables us to select the appropriate multiplexer, to minimize the number of ICs needed to implement a given logic function.

For example, if data is to be selected from two 16-bit sources, it is more convenient to use 2-input multiplexers, than 4-input or 8-input multiplexers.

Some additional features:

• Higher drive capability of a multiplexer enables the designer to save on buffers and the consequent delay in certain situations.
• Availability of complementary outputs often results in the saving of additional inverters.
• Availability of tri-state outputs make it easy to tie the outputs of a number of multiplexers without using additional gates.

There are two propagation delays that are of interest to designers:

• Delay from the data inputs to the output
• Delay from the select input to the output

These timing relationships are shown in figure below.

Timing relationship between signals of a multiplexer

Data Selection

The multiplexer was mainly designed for selecting data from several sources. For example, if we are required to select an 8-bit data from one of four possible sources, then, it can be realized through four dual 4-input multiplexers, like 74LS153. The circuit that realizes such a data selection is shown in figure below.

Circuit for selecting 8-bit data from four sources

Another conventional use of the multiplexers is one of time-division gating of several data lines on to a transmission channel using SELECT lines. This is done by using a `Multiplexer' as the sending unit, and a `Demultiplexer' as a receiving unit. The sending end of such a transmission system which multiplexes 32 data lines is shown in Figure below.

1-of-32 data selector