Facebook
Google
GitHub
LinkedinDigital Circuits
Multiplexers and Demultiplexers
15 questions By Tony R. Kuphaldt
-
Question 10 of 15
The 74HC137 and 74HC237 decoder/demultiplexer integrated circuits have a feature that some other decoder/demultiplexers do not: address latching. Explain what this additional feature is, how it works, and how you would disable the feature if you needed to use one of these integrated circuits in an application not requiring address latching.
Reveal answerI’ll let you figure out the answers to this question. The manufacturers’ datasheet, of course, is probably your best source of information!
Follow-up question: what is the functional difference between the 74HC137 and the 74HC237? How would the respective schematic symbols for these two decoder/demux ICs differ?
Challenge question: think of a practical application where you might need the feature of address latching.
Notes:The feature of address latching is not hard to understand if students have already studied D-type latches (and/or parallel-in/parallel-out shift registers).
-
Question 11 of 15
When first learning about encoders, decoders, multiplexers, and demultiplexers, students often get them confused with one another. Write succinct definitions (complete with illustrations) for each of these four digital functions, based on your own research and written using your own words. Also, identify which two of these digital functions are usually performed by the same integrated circuit.
Reveal answerI’ll let you figure out the first part of the answer (four succinct definitions, complete with illustrations)! Because it is so easy to get these four functions mixed up in your mind, it is important that you arrive at your own understanding of which is which, rather than have someone else (like me) provide ready-made definitions for you to memorize.
Decoding and demultiplexing are usually performed by the same integrated circuit device. A good example of this is the 74HC154.
Notes:Here is a good example of where students like to fall on rote memorization, and where such shallow learning strategies often fail. It is imperative that students do their own research and cast these functions into their own personal terms. This way, they will understand the differences rather than just memorize the differences.
Call on individual students to present their findings for this question, and let the classroom be a place where students share their understandings with one another. Let them know that this is important for them to grasp, but do not simply provide ready-made answers for them!
-
Question 12 of 15
The following schematic diagram shows an eight-step arbitrary waveform generator. The analog multiplexer selects one of the eight potentiometer signals at a time, stepping from one to the next at the pace of the clock pulse:
Explain what effect a shorted bilateral switch would have on the output waveform. Be as specific as possible.
Reveal answerIf one of the bilateral switches were to fail shorted, it would skew all the arbitrary waveform voltages “toward” the one with the failed switch, distorting the waveform from its original shape.
Notes:A failed-shorted bilateral switch would create a sort of passive averager circuit between the shorted-switch pot and the selected-switch pot. If your students have difficulty figuring out the effects of this fault, remind them of what a passive averager is, and how it works.
