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?

Difficulty: Easy

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

More Questions from Combinational Logic Circuits

Discussion & Comments

No comments yet. Be the first to comment!
Join Discussion