UGC-NET Computer Science December 2004 Answer Key with Explanation

Q:: 6    An example of a binary number which is equal to its 2’s complement is:

(A) 1100                        (B) 1001

(C) 1000                        (D) 1111

Answer: C

Explanation:

2's complement of (A) 1100 is 0100
2's complement of (B) 1001 is 0111
2's complement of (C) 1000 is 1000
2's complement of (D) 1111 is 0001

To calculate 2's complement of a binary number start from right side and copy down all 0's and

the first encountered 1, after that complement all subsequent bits 0 to 1 and 1 to 0.

Q:: 7   When a tri-state logic device is in the third state, then:
         (A) it draws low current 
         (B) it does not draw any current
         (C) it draws very high current
         (D) it presents a low impedance

Answer: D

Explanation:

In digital electronics three-state, tri-state, or 3-state logic allows an output port to assume a high impedance state in addition to the 0 and 1 logic levels, effectively removing the output from the circuit.
This allows multiple circuits to share the same output line or lines (such as a bus which cannot listen to more than one device at a time).Three-state outputs are implemented in many registers, bus drivers, and flip-flops in the 7400 and 4000 series as well as in other types, but also internally in many integrated circuits. Other typical uses are internal and external buses in microprocessors, computer memory, and peripherals. Many devices are controlled by an active-low input called OE(Output Enable) which dictates whether the outputs should be held in a high-impedance state or drive their respective loads (to either 0- or 1-level).

A tri-state buffer can be thought of as a switch. If B is on, the switch is closed. If B is off, the switch is open.         

INPUT		OUTPUT
A	B	C
0	1	0
1		1
X	0	Z (high impedance)

Q:: 8   An example of a connective which is not associative is:

(A) AND             (B) OR

(C) EX-OR         (D) NAND

Answer: D

Explanation:
This can be proved by truth table::

            
As can be seen by inspection, the truth values under the main connectives do not match for all boolean interpretations.

Q:: 9    Essential hazards may occur in:

(A) Combinational logic circuits

(B) Synchronous sequential logic circuits

(C) Asynchronous sequential logic circuits working in the fundamental mode

(D) Asynchronous sequential logic circuits working in the pulse mode

Answer: C

Q::10.    The characteristic equation of a T flip-flop is:

   (A) Q_n+1=TQ’_n + T’ Q_n              

   (B) Q_n+1=T+Q_n

   (C) Q_n+1=TQ_n                            

   (D) Q_n+1= T’Q’_n

   The symbols used have the usual meaning.

Answer: A

Explanation:

FLIP-FLOP NAME	FLIP-FLOP SYMBOL	CHARACTERISTIC TABLE	CHARACTERISTIC EQUATION	EXCITATION TABLE
SR		S R Q(next) 0 0 Q 0 1 0 1 0 1 1 1 ?	Q(next) = S + R'QSR = 0	Q Q(next) S R 0 0 0 X 0 1 1 0 1 0 0 1 1 1 X 0
JK		J K Q(next) 0 0 Q 0 1 0 1 0 1 1 1 Q'	Q(next) = JQ' + K'Q	Q Q(next) J K 0 0 0 X 0 1 1 X 1 0 X 1 1 1 X 0
D		D Q(next) 0 0 1 1	Q(next) = D	Q Q(next) D 0 0 0 0 1 1 1 0 0 1 1 1
T		T Q(next) 0 Q 1 Q'	Q(next) = TQ' + T'Q	Q Q(next) T 0 0 0 0 1 1 1 0 1 1 1 0

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