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Encoders and Decoders
21 questions By Tony R. Kuphaldt
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Question 7 of 21
Explain how the following decimal-to-BCD encoder circuit works:
Also, determine which output (D or A) is the most significant bit of the BCD output.
Reveal answerI’ll let you figure out the details of how this circuit functions, but I will tell you that D is the MSB.
Follow-up question: what would have to be modified in order to turn this into a hexadecimal-to-binary (16 line to 4 line) encoder? Would any additional OR gates have to be added to the circuit?
Notes:Non-priority encoder circuits such as this one are fairly simple to figure out, and so I do not provide an explanation for students in the “Answer” section.
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Question 8 of 21
Suppose OR gate U3 were to fail with the output terminal always high. Which output codes would be affected by this fault?
Reveal answerThe following output codes would be adversely affected by this fault: 1, 4, 5, 8, and 9.
Notes:Ask your students to explain how they determined the affected codes. Although the question itself is relatively simple, there is room for creativity in the answers!
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Question 9 of 21
Here is the block symbol for the 74HC147 decimal-to-BCD encoder:
Describe what sort of input conditions would be required to make it generate the code for the number 7, and how that numerical quantity would be represented on the output (Y) lines.
Reveal answerTo encode the number 7, you would have to make the I7 line low (connect it to ground). This would make the outputs assume the following states:
- Y0 = low
- Y1 = low
- Y2 = low
- Y3 = high
Notes:This is a good review of active-low inputs, how they are drawn on schematics, and what they mean in practical digital circuits. A potentially confusing aspect of this question is the presence of active-low inputs and outputs, but it is well worth your time to review with students, because like it or not there are many ICs with active-low I/O lines.
