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Multiplexers and Demultiplexers


Multiplexers and demultiplexers are used in serial data transmission systems where a single transmission line can be used to transmit information in a large number of data lines by letting each data line have access to the transmission line on a time-share basis, Figure 40.

Figure 40: Multiplexer-demultiplexer time-share arrangement.

Multiplexers selectively connect one and only one of 2n input lines Ii to the transmission line Y.

Example. Here is a truth table defining a 4-1 MUX:

EN S1 S0 Y
0 X X 0
1 0 0 I0
1 0 1 I1
1 1 0 I2
1 1 1 I3

\begin{displaymath}Y = \overline{S}_1 \overline{S}_0 I_0 EN
+ {S}_1 \overline{S}_0 I_1 EN
+ \overline{S}_1 {S}_0 I_2 EN
+ {S}_1 {S}_0 I_3 EN

Example. Multiplexers can be used to implement combinational logic functions. Consider the cannonical SOP expression

\begin{displaymath}F(A,B,C) = \sum m(1,3,4,7) .

This function is implemented by a MUX in Figure 41.

Figure: MUX used to implement the logic function $F =\sum m(1,3,4,7)$.

There are no specific demultiplexers, since the DMUX function can be implemented using a decoder.

Exercise. Show how a DMUX can be built from a decoder.

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