This set of Logic Design Multiple Choice Questions & Answers (MCQs) focuses on “Design of Sequential Circuits Using ROMs and PLAs”.
1. Which operation of the following is performed by ROMs in a sequential circuit?
a) Sequential operation
b) Combinational operation
c) Logical operation
d) Mathematical operation
View Answer
Explanation: A ROM circuit contains an m × n matrix configuration of diodes or transistors to hold data that can be used as data selectors. Hence the right answer is combinational operation.
2. A ROM circuit contains 8 input lines. How many words can it contain?
a) 16
b) 256
c) 128
d) 1024
View Answer
Explanation: ROM defines Read Only Memory. It is a type of non-volatile memory. A ROM of k inputs can contain 2k words. Hence the number of words it can contain is 28 = 256.
3. What is the full form of PLA?
a) Programmed Array Logic
b) Programmable Logic Array
c) Programmed Logical Array
d) Programmable Array Logic
View Answer
Explanation: PLA stands for Programmable Logic Array. PLAs have a set of AND gate planes which link to a plane of OR gates to implement logic equations. These devices are often used to replace ROMs in several circuits.
4. How is the digital words constructed inside a ROM circuit?
a) By connecting individual LEDs along the column lines
b) By connecting diodes at the row and column junction where logic 1s are needed and keeping the others open
c) By directly connecting the row and column line at the row column junction
d) By open circuiting the row-column junction where digital 0s are needed
View Answer
Explanation: Inside the ROM circuit, the digital 1s are constructed by connecting the row and column junctions by diodes to make a connection between rows and columns. Thus any specific word of combination of 1s and 0s is selected just by applying the column number.
5. A BCD to Excess 3 converter is designed using a PROM circuit. How many words does the PROM contain?
a) 4
b) 8
c) 16
d) 32
View Answer
Explanation: A BCD to Excess 3 converter must have 4 inputs as BCD code always represents a digit in 4bit binary code. Thus a 4 input ROM will contain 24 = 16 words. Hence the right answer is 16.
6. Let a ROM is to be used in a binary to gray code converter circuit. To get the gray code of the input 11011. What will be the row and column number of the ROM?
a) 16 rows, 16 columns
b) 5 rows, 16 columns
c) 32 rows, 5 columns
d) 5 rows, 32 columns
View Answer
Explanation: As the input is of 5 bit, we need a 5 bit input port. Thus 5 input lines will specify 25 = 32 words i.e. 32 rows. As the output length will also be 5 bit, the number of columns will also be 5.
6. Which of the following components is not used in logic designing using ROM?
a) Resistors
b) Bipolar Junction Transistors
c) Diodes
d) MOSFTEs
View Answer
Explanation: BJTs, MOSFETs and diodes are generally used to make a connection between the row and column lines where required. But resistors are not used.
7. Which of the following symbols define OR operation in PLA designing?
a) ×
b) o
c) □
d) °
View Answer
Explanation: PLA or Programmable Logic Arrays are used to implement logic functions. PLAs contain sets of AND gates and OR gates to implement a specific logic. The square symbol signifies the OR operation. ‘x’ defines AND operation. Rest of the symbols are not used in PLA design.
8. ROM has the capability to perform which of the following operations?
a) Write operation only
b) Read operation only
c) Both write and read operation
d) Erase operation
View Answer
Explanation: ROM means “Read Only Memory”. Hence, it has the capability to perform read operation only. No write or erase operation could be performed in the ROM.
9. Who stores the necessary information in computer ROM?
a) User
b) Programmer
c) Manufacturer
d) Hardware Designer
View Answer
Explanation: ROM means “Read Only Memory”. Hence, it has the capability to perform read operation only. No write or erase operation could be performed in the ROM. ROM contains necessary information and programs for booting the computer. These important programs segments are loaded to ROM by the manufacturer by the time of manufacturing.
10. How many times a PROM can be programmed?
a) Never
b) Twice
c) Once only
d) As many times needed
View Answer
Explanation: PROM or programmable ROM (programmable read-only memory) is a computer memory chip that can be programmed once after it is created. Once the PROM is programmed, the information written is permanent and cannot be erased or deleted. PROM was first developed by Wen Tsing Chow in 1956.
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