GATE 2020 IN – Question 22

Digital Electronics · Number systems and combinational logic: Number systems, multiplexer and demultiplexer, truth tables, minimization of Boolean functions, IC families (ECL, TTL, CMOS), CMOS logic gates, arithmetic circuits · 1 mark · Multiple choice

For the shown 4:1 multiplexer with select bits $S_1$ (MSB) and $S_0$ (LSB), the output O is

$S_1\oplus S_0$
$S_1\bar S_0$
$S_1+S_0$
$S_0\bar S_1$

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Show answer and explanation

Correct answer: (A) $S_1\oplus S_0$

Explanation

The data inputs, after following the two inverters and ground connections, are $I_0=0,I_1=1,I_2=1,I_3=0$. The selected output for select words 00,01,10,11 is therefore 0,1,1,0. That is the exclusive-OR truth table, $O=S_1\oplus S_0$, A.

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