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Demultiplexers (Distributors)



A demultiplexer can be used to distribute input information d to various outputs. It represents an extension of the 1-out-of-n decoder. The addressed output does not go to "one", but assumes the value of input variable d. Figure 1.43 illustrates the principle by means of switches, Fig. 1.44. shows its implemen­tation using gates. If we make d = const = 1, the multiplexer operates as a 1-out-of-n decoder. Commonly used demultiplexers are listed in Fig. 1.45

Fig. 1.43 - Basic mode of operation. Fig. 1.44 - Demultiplexer circuit.




74LS138 74HC138
74ALS5381   40H.138
2 x 4 74LS139 74HC139
2x4 74ALS5391  

1 Output polarity reversible


Fig. 1.45 - Integrated demultiplexers.


A 2n -output demultiplexer is a combinational system with n control (select) inputs s=(sn-1, . . . , s0), one data input x, a module-enable input E, and 2n data outputs y = ( , . . . , y0) (Figure 1.46.). It performs the inverse operation of the multiplexer: the input is routed to the output selected by the select variables. All other outputs are zero.


Fig. 1.46 - n-input demultiplexor.


A high-level description is


yi= x if i=s and E= 1
0 if i ¹s and E=0





In the description of systems this module is denoted as


y = DMX(x,s,E)


For example, DMX(1, (1,0,1),1)=(0,0,1,0,0,0,0,0).

The switching expressions representing the outputs are



These expressions are similar to those of the decoder, the difference being the additional data input in the demultiplexer case. Consequently, a demultiplexer can be used as a decoder. The use of a decoder as a demultiplexer is also possible provided that the enable input of the decoder is used as the data input of the demultiplexer. Of course, in this last situation the resulting demultiplexer would not have an enable input. A vector demultiplexer can be defined in a manner similar to a vector multiplexer.

The basic use of a demultiplexer is to transmit the input to one out of several outputs under control of the select inputs.


Date: 2015-01-12; view: 1304

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