# Digital Circuits Questions and Answers – Shift Register Counters

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

1. What is a recirculating register?
a) Serial out connected to serial in
b) All Q outputs connected together
c) A register that can be used over again
d) Parallel out connected to Parallel in

Explanation: A recirculating register is a register whose serial output is connected to the serial input in a circulated manner.

2. When is it important to use a three-state buffer?
a) When two or more outputs are connected to the same input
b) When all outputs are normally HIGH
c) When all outputs are normally LOW
d) When two or more outputs are connected to two or more inputs

Explanation: When two or more outputs are connected to the same input, in such situation we use of tristate buffer always because it has the capability to take upto three inputs. A buffer is a circuit where the output follows the input.

3. A bidirectional 4-bit shift register is storing the nibble 1110. Its input is LOW. The nibble 0111 is waiting to be entered on the serial data-input line. After two clock pulses, the shift register is storing ________
a) 1110
b) 0111
c) 1011
d) 1001

Explanation: Given,

```Waiting nibble | Stored nibble
0111 | 1110, Initially
011  | 1101, 1st pulse (left shift)
01   | 1011, 2nd pulse (left shift)
```

4. In a parallel in/parallel out shift register, D0 = 1, D1 = 1, D2 = 1, and D3 = 0. After three clock pulses, the data outputs are ________
a) 1110
b) 0001
c) 1100
d) 1000

Explanation: Parallel in parallel out gives the same output as input. Thus, after three clock pulses, the data outputs are 0001.

5. The group of bits 10110111 is serially shifted (right-most bit first) into an 8-bit parallel output shift register with an initial state 11110000. After two clock pulses, the register contains ______________
a) 10111000
b) 10110111
c) 11110000
d) 11111100

Explanation: After first clock pulse, the register contains 11111000. After second clock pulse, the register would contain 11111100. Since the bits are shifted to the right at every clock pulse.

6. By adding recirculating lines to a 4-bit parallel-in serial-out shift register, it becomes a ________ ________ and ________ out register.
a) Parallel-in, serial, parallel
b) Serial-in, parallel, serial
c) Series-parallel-in, series, parallel
d) Bidirectional in, parallel, series

Explanation: One bit shifting takes place just after the output obtained on every register. Hence, by adding recirculating lines to a 4-bit parallel-in serial-out shift register, it becomes a Parallel-in, Serial, and Parallel-out register. Since, the bots can be inputted all at the same time, while the data can be outputted either one at a time or simultaneously.
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7. What type of register would have a complete binary number shifted in one bit at a time and have all the stored bits shifted out one at a time?
a) Parallel-in Parallel-out
b) Parallel-in Serial-out
c) Serial-in Serial-out
d) Serial-in Parallel-out

Explanation: Serial-in Serial-out register would have a complete binary number shifted in one bit at a time and have all the stored bits shifted out one at a time. Since in serial transmission, bits are transmitted or received one at a time and not simultaneously.

8. In a 4-bit Johnson counter sequence, there are a total of how many states or bit patterns?
a) 1
b) 3
c) 4
d) 8

Explanation: In johnson counter, total number of states are determined by 2N = 2*4 = 16
Total Number of Used states = 2N = 2*4 = 8
Total Number of Unused states = 16 – 8 = 8.

9. If a 10-bit ring counter has an initial state 1101000000, what is the state after the second clock pulse?
a) 1101000000
b) 0011010000
c) 1100000000
d) 0000000000

Explanation: After shifting 2-bit we get the output as 0011010000 (Since two zeros are at 1st position and 2nd position which came from the last two bits). As in a ring counter, the bits rotate in clockwise direction.

10. How much storage capacity does each stage in a shift register represent?
a) One bit
b) Two bits
c) Four bits
d) Eight bits

Explanation: A register is made of flip-flops. And each flip-flop stores 1 bit of data. Thus, a shift register has the capability to store one bit and if another bit is to store, in such a situation it deletes the previous data and stores them.

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