# Digital Circuits Questions and Answers – Universal Shift Registers

This set of Digital Electronics/Circuits Multiple Choice Questions & Answers (MCQs) focuses on “Universal Shift Registers”.

1. A sequence of equally spaced timing pulses may be easily generated by which type of counter circuit?
a) Ring shift
b) Clock
c) Johnson
d) Binary

Explanation: In Ring counter, the feedback of the output of the FF is fed to the same FF’s input. Thus, it generates equally spaced timing pulses.

2. A bidirectional 4-bit shift register is storing the nibble 1101. Its input is HIGH. The nibble 1011 is waiting to be entered on the serial data-input line. After three clock pulses, the shift register is storing ________
a) 1101
b) 0111
c) 0001
d) 1110

Explanation: Mode is high means it’s a right shift register. Then after 3 clock pulses enter bits are 011 and remained bit in register is 1. Therefore, 0111 is the required solution.
1011 | 1101
101 | 1110 -> 1st clock pulse
10 | 1111 -> 2nd clock pulse
1 | 0111 -> 3rd clock pulse.

3. To operate correctly, starting a ring shift counter requires __________
a) Clearing all the flip-flops
b) Presetting one flip-flop and clearing all others
c) Clearing one flip-flop and presetting all others
d) Presetting all the flip-flops

Explanation: In Ring counter, the feedback of the output of the FF is fed to the same FF’s input. To operate correctly, starting a ring shift counter requires presetting one flip-flop and clearing all others so that it can shift to the next bit.

4. A 4-bit shift register that receives 4 bits of parallel data will shift to the ________ by ________ position for each clock pulse.
a) Right, one
b) Right, two
c) Left, one
d) Left, three

Explanation: If register shifts towards left then it shift by a bit to the left and if register shifts right then it shift to the right by one bit. Since, it receives parallel data, then by default, it will shift to right by one position.

5. How many clock pulses will be required to completely load serially a 5-bit shift register?
a) 2
b) 3
c) 4
d) 5

Explanation: A register is a collection of FFS. To load a bit, we require 1 clock pulse for 1 shift register. So, for 5-bit shift register we would require of 5 clock pulses.

6. How is a strobe signal used when serially loading a shift register?
a) To turn the register on and off
b) To control the number of clocks
c) To determine which output Qs are used
d) To determine the FFs that will be used

Explanation: A strobe is used to validate the availability of data on the data line. It (an auxiliary signal used to help synchronize the real data in an electrical bus when the bus components have no common clock) signal is used to control the number of clocks during serially loading a shift register.

7. An 8-bit serial in/serial out shift register is used with a clock frequency of 150 kHz. What is the time delay between the serial input and the Q3 output?
a) 1.67 s
b) 26.67 s
c) 26.7 ms
d) 267 ms

Explanation: In serial-shifting, one bit of data is shifted one at a time. From Q0 to Q3 total of 4 bit shifting takes place. Therefore, 4/150kHz = 26.67 microseconds.

8. What are the three output conditions of a three-state buffer?
a) HIGH, LOW, float
b) High-Z, 0, float
c) Negative, positive, 0
d) 1, Low-Z, float

Explanation: Three conditions of a three-state buffer are HIGH, LOW & float.

9. The primary purpose of a three-state buffer is usually ____________
a) To provide isolation between the input device and the data bus
b) To provide the sink or source current required by any device connected to its output without loading down the output device
c) Temporary data storage
d) To control data flow

Explanation: The primary purpose of a three-state buffer is usually to provide isolation between the input device or peripheral devices and the data bus. Three conditions of a three-state buffer are HIGH, LOW & float.

10. What is the difference between a ring shift counter and a Johnson shift counter?
a) There is no difference
b) A ring is faster
c) The feedback is reversed
d) The Johnson is faster

Explanation: A ring counter is a shift register (a cascade connection of flip-flops) with the output of the last one connected to the input of the first, that is, in a ring. Whereas, a Johnson counter (or switchtail ring counter, twisted-ring counter, walking-ring counter, or Moebius counter) is a modified ring counter, where the output from the last stage is inverted and fed back as input to the first stage.

Sanfoundry Global Education & Learning Series – Digital Circuits.

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