Refer to the keyboard encoder in figure (a). Sometimes when the 5 key is pressed

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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.

Correct Answer

The switches on the 5 key are intermittent; the contacts need to be cleaned or the switch replaced.

MSI Logic Circuits problems

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1. 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.

3. 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.

4. 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.

5. Refer to the figure given below. The logic function generator being implemented with the multiplexer in this circuit produces a constant LOW on the output. The ABC inputs are checked and appear to be pulsing; also, the 0?7 and EN inputs are checked with the scope and all appear to be at 0 V. A check with the DMM confirms that power is on. What is the problem, and what should be done to correct it?
Options
A. The output is shorted to V_cc; replace the IC.
B. The scope's vertical input is in the AC mode and the common connection for the 0,2,3 and 5 inputs is bad. Set the scope's vertical input mode to DC, and repair the bad solder connection.
C. Power has not been applied to the circuit; apply power.
D. The output is shorted to ground; replace the IC.

8. 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

9. 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

10. 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.

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