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How can I combine, for example, a number of 3-8 demux units and perhaps other components such as transistors (takes three inputs and has 8 outputs) in order to make a 4-16 demux unit or perhaps even a 5-32 demux unit? How do I use the extra control bits that do not go into the 3-8 subunit in order to obtain a bigger demux?

By a demux unit I mean something like a http://www.nxp.com/documents/data_sheet/74HC_HCT238.pdf -- a demultiplexer

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  • \$\begingroup\$ What are you talking about? \$\endgroup\$ – Transistor Dec 20 '15 at 19:31
  • \$\begingroup\$ Think about how you can select the demux you need with the extra pins you have. Do you have an enable pin to work with? In that case this is pretty simple. \$\endgroup\$ – Daniel Dec 20 '15 at 19:32
  • \$\begingroup\$ So for example I would use the 4th control bit to enable or disable one of the demux'es in the pair to obtain 16 outputs? \$\endgroup\$ – andreas.vitikan Dec 20 '15 at 19:33
  • \$\begingroup\$ Yes, use the Enable inputs to select which mux is active. Note that there are two active-Low and one active-High enable inputs, to give some flexibility in using multiple '238s together without requiring extra parts. \$\endgroup\$ – Peter Bennett Dec 20 '15 at 19:59
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for 4-16 use the Most Significant I/P line as a select line to separate two 3 to 8 decoders. When it is 0 one of lower significance is enabled when 1 then other. For 74HC238 used as lower decoder take three dummy lines with logic 1 and "AND" two them with most significant I/P (here selection line say s1) to form connection of not(E1) and not(E2) and use "nand" with E3 to form logic.

Note that when select line is high it implies decoder with higher significance is activated and vica versa is also true.

Similar procedure may be followed with the decoder with higher significance.

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