Output 5 of a 74138 octal decoder is selected when it is enabled by a data input

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Output 5 of a 74138 octal decoder is selected when it is enabled by a data input of:
Options
A.
B.
C.
D.

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1. Refer to the keyboard encoder in figure (a). Sometimes when the 5 key is pressed, the system attached to the keypad does not respond. The 5 input on the 74147 is monitored with a digital storage scope while repeatedly pressing the 5 key, and the waveform in figure (b) is obtained; the P above the trace indicates the points at which the technician pressed the key. What is most likely wrong with the circuit?
Options
A. The switches on the 5 key are intermittent; the contacts need to be cleaned or the switch replaced.
B. The pull-up resistor connected to the 5 key is bad and should be replaced.
C. The common ground connection at the bottom of the 0 key has a bad solder connection; repair the connection.
D. The 74147 is intermittent, possibly due to high temperature, and should be replaced.

2. It is suspected that the comparator in the figure given below has a problem. The inputs are activated in the table shown below and the corresponding outputs noted. What is most likely wrong with the circuit?
For P0 ? P3 = 1 and Q0 ? Q3 = 0, P > Q = 1, P = Q = 1, P < Q = 0
For P0 ? P3 = 0 and Q0 ? Q3 = 1, P > Q = 0, P = Q = 1, P < Q = 1
For P0 ? P3 = 1 and Q0 ? Q3 = 1, P > Q = 0, P = Q = 1, P < Q = 0
Options
A. The P0?P3 inputs are defective.
B. One or more of the Q inputs is bad.
C. The P = Q output is shorted to V_cc.
D. Nothing is wrong; the circuit is functioning properly.

4. What is the purpose of a decoder's inputs?
Options
A. To allow the decoder to respond to the inputs to activate the correct output gate.
B. To disable the decoder outputs so that all outputs will be inactive.
C. To disable the inputs and activate all outputs.
D. To allow the decoder to respond to the inputs to activate the correct output gate, and to disable the inputs and activate all outputs.

5. Which statement best describes the given figure, and what is the function of the terminal labeled EN?
Options
A. Quad two-input multiplexer. EN is the enable input, which requires an active LOW for the device to work.
B. Quad two-bit multiplier, EN is the active HIGH trigger.
C. Dual quad-input multiplexer, which requires an active LOW on the EN terminal for the device to work.
D. Quad two-input AND gate, which requires an active LOW on the EN input to enable all the gates.

7. What type of device is shown in the given figure, and what inputs (A3,A2,A1,A0) are required to produce the output levels as shown?
Options
A. A binary-to-decimal encoder; 0,1,1,1
B. A decimal-to-binary decoder; 1,1,1,0
C. A BCD-to-decimal decoder; 0,1,1,1
D. A decimal-to-BCD encoder; 1,1,1,0

8. What is an important attribute of the conditional signal assignment statement?
Options
A. Its tristate outputs
B. Its sequential evaluation
C. Its use of library components
D. Its fast activation times

9. The IEEE/ANSI symbol for a decoder has the internal designation bcd/dec. This means the decoder is a:
Options
A. decimal-to-BCD decoder with ten inputs and four outputs.
B. BCD-to-decimal decoder with ten inputs and four outputs.
C. decimal-to-BCD decoder with four inputs and ten outputs.
D. BCD-to-decimal decoder with four inputs and ten outputs.

10. What VHDL techniques are used to describe a priority encoder?
Options
A. Integer outputs and priority coding
B. Signal outputs and priority coding
C. Tristate outputs and priority coding
D. Variables and priority coding

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