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?
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Aseveral SSI logic gates
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Bcombinational logic circuits
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Cseveral Ex-NOR gates
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Dseveral SSI logic gates or combinational logic circuits
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Ea 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