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Multiplexers and Demultiplexers
15 questions By Tony R. Kuphaldt
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Question 7 of 15
Multiplexers, or data selectors, may be used to generate arbitrary truth table functions. Take for example this Boolean SOP expression, shown beside a symbol for a 16-channel multiplexer:
Show the wire connections necessary to make the multiplexer output the specified logic states in response to the data select (A, B, C, and D) inputs.
Reveal answer
Notes:Discuss with your students the significance of using a multiplexer in this fashion: to implement arbitrary logic functions. For those students who may not be familiar with the term, be sure to define the word “arbitrary.” It may seem silly, but students often fail to ask for the definitions of words that are new to them, for fear of sounding dumb in front of their peers and in front of you. One more reason to model respect in your classroom, and also to create an atmosphere where students feel comfortable asking any question.
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Question 8 of 15
The 74HC154 is a high-speed CMOS (TTL-compatible) integrated circuit decoder with four input lines and sixteen output lines:
Identify the logic states of all output terminals given the input conditions shown.
Now, identify the logic states for the same circuit, this time with a square wave (on/off pulse) logic signal applied to the enable terminals:
Reveal answerIn the first scenario with the enable lines grounded, output #11 will be low, with all other outputs high. In the second scenario with a square wave signal input to the enable lines, output #11 will pulse while all other outputs remain in the high state.
Notes:This question previews the use of a decoder as a demultiplexer.
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Question 9 of 15
The 74HC154 is a high-speed CMOS (TTL-compatible) integrated circuit decoder, which may also be used as a demultiplexer:
What terminal(s) do we use for the signal input, if using this chip as a demultiplexer and not just a decoder?
Reveal answerUse the enable terminals (together) as one “input” terminal.
Notes:Explain to your students that “decoder” and “demultiplexer” are really just two different words for the same device, seeing as how all real decoders come equipped with enable inputs.
