More Questions from Combinational Logic Circuits

In digital communications and error detection circuits, parity generators and parity checkers are commonly implemented using specific logic gates. Which type(s) of logic gates are typically used to generate a parity bit and to check parity (even or odd) in such systems?

Electronics Combinational Logic Circuits Difficulty: Easy
Choose an option
  • A
    Exclusive-OR (XOR) gates only
  • B
    Exclusive-OR (XOR) and Exclusive-NOR (XNOR) gates
  • C
    Exclusive-NAND gates only
  • D
    Exclusive-AND gates

Answer

Correct Answer: Exclusive-OR (XOR) and Exclusive-NOR (XNOR) gates

Explanation

Introduction / Context: Parity is one of the simplest and most widely used error-detection techniques in digital systems, serial links, memory subsystems, and communication protocols. A parity generator produces a single parity bit that makes the total number of 1s either even (even parity) or odd (odd parity). A parity checker then verifies that property on reception. Understanding which logic primitives naturally realize these functions is essential in combinational logic design and exam preparation.Given Data / Assumptions:

  • Task: identify the gate type(s) used for parity generation and checking.
  • Even parity requires an even count of 1s; odd parity requires an odd count.
  • We assume standard two-level logic without encoded tricks.

Concept / Approach: The XOR operation outputs 1 if and only if the number of 1s at its inputs is odd. Cascading XORs across multiple bits therefore produces an output that represents the odd-parity function. Conversely, XNOR outputs 1 if and only if the number of 1s is even. Thus, XOR naturally generates odd parity and can be inverted for even parity; XNOR naturally generates even parity and can be inverted for odd parity. Parity checkers compare the received data bits and parity bit using XOR or XNOR to verify integrity.Step-by-Step Solution:

For N data bits d0, d1, …, dN-1, compute P = d0 XOR d1 XOR … XOR dN-1.If odd parity is required, transmit parity_bit = P; for even parity, use parity_bit = P’ or use XNOR across all bits.At the receiver, XOR all bits including the parity bit; a 0 result indicates even parity satisfied (or choose XNOR with the corresponding rule).

Verification / Alternative check:

Truth-table inspection for two inputs shows XOR counts odd ones; XNOR counts even ones. Cascading preserves this count property for more bits.

Why Other Options Are Wrong:

Exclusive-OR (XOR) gates only: Parity checking often uses XNOR directly for even-parity validation.Exclusive-NAND gates only / Exclusive-AND gates: These are not standard parity primitives and do not map directly to odd/even count logic.

Common Pitfalls:

Confusing XOR with OR; XOR is parity-sensitive whereas OR is not.Forgetting that XNOR is the complement parity function (even count detector).

Final Answer:

Exclusive-OR (XOR) and Exclusive-NOR (XNOR) gates
Discussion & Comments
No comments yet. Be the first to comment!
Join Discussion