Digital Electronics - MSI Logic Circuits

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"Example is better than precept."
- (Proverb)

Which statement below best describes the function of a decoder?

A. A decoder will convert a decimal number into the proper binary equivalent.
B. A decoder will convert a binary number into a specific output representing a particular character or digit.
C. Decoders are used to prevent improper operation of digital systems.
D. Decoders are special ICs that are used to make it possible for one brand of computer to talk to another.


One can safely state that the output lines for a demultiplexer are under the direct control of the:

A. input data select lines.
B. the internal AND gates.
C. the internal OR gate.
D. Input data line.


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?

A. The output is shorted to Vcc; 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.


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?

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.


The BCD/DEC decoder shown in figure (a) is examined with a logic analyzer and the results are shown in the waveforms in figure (b). What, if anything, is wrong with the circuit?

A. The "2" output is shorted to Vcc.
B. The A1 input is internally open.
C. The A1 input node is internally stuck LOW.
D. Nothing is wrong with the circuit.

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