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
Best Answer
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.