Functional replacement by a multiplexer (MUX) In digital design, a single multiplexer can effectively take the place of which of the following elements when used to implement an arbitrary truth table or selection logic?

Digital Electronics Code Converters and Multiplexers Difficulty: Easy
Choose an option
  • A
    several SSI logic gates
  • B
    combinational logic circuits
  • C
    several Ex-NOR gates
  • D
    several SSI logic gates or combinational logic circuits
  • E
    a counter with built-in decoder

Answer

Correct Answer: several SSI logic gates or combinational logic circuits

Explanation

Introduction / Context:Multiplexers (MUXes) are universal combinational building blocks. By wiring data inputs to logic 0/1 (or signals) and steering with select lines, a MUX can implement many logic functions that would otherwise require multiple small-scale integration (SSI) gates. Understanding this equivalence helps minimize chip count and PCB area.

Given Data / Assumptions:

  • A standard n-to-1 multiplexer is available (e.g., 4:1, 8:1, 16:1).
  • We can tie MUX inputs to constants or real signals.
  • Select lines represent address bits of a truth table.

Concept / Approach:Any truth table with up to n selectable minterms can be realized by mapping select lines to the table’s variables and data inputs to the desired output values for each address. This directly replaces networks of AND/OR/NOT gates used in sum-of-products or other combinational forms.

Step-by-Step Solution:

Choose MUX size so that 2^k data inputs ≥ number of minterms to implement.Connect select lines to k input variables.Wire each data input to 0, 1, or a signal such that the MUX output equals the truth table for that address.Result: the MUX replaces many SSI gates or an entire combinational subcircuit.

Verification / Alternative check:Compare gate counts: implementing a 4-variable logic function via canonical SOP may require numerous gates, whereas a single 16:1 MUX plus inverters on select lines can implement the same function compactly and predictably.

Why Other Options Are Wrong:

  • “Several Ex-NOR gates”: overly specific; a MUX can emulate them but also far more.
  • Only “SSI gates” or only “combinational circuits” is incomplete. The best description includes both.
  • Counters are sequential devices; a MUX cannot replace their state storage.

Common Pitfalls:

  • Ignoring the need for input inversion; sometimes inverters on select lines are required.
  • Overlooking propagation delay and fan-out when replacing large gate networks.

Final Answer:several SSI logic gates or combinational logic circuits

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