How it works
The 74151 has eight data inputs D0–D7, three select lines, an active-low enable, and complementary outputs Y and Y-bar. When S2S1S0 = 011, only D3 is connected to output Y; the binary value on the select lines acts as the address of the input channel. The output expression is Y = Σ(mi · Di) where mi is the minterm corresponding to the select combination. With a 5V supply and propagation delay around 22 ns, the 74151 can handle data rates well above 1 MHz before timing becomes a concern.
Key points to remember
A 2ⁿ-to-1 MUX needs n select lines — so a 16-to-1 MUX requires 4 select lines. The 74151 implements any 3-variable Boolean function directly by connecting the function's truth table values to D0–D7. For a 4-variable function, one variable is applied to the enable or a data input while the other three drive the select lines. Propagation delay of standard TTL 74151 is typically 22 ns. Cascading two 8-to-1 MUXes with a 2-to-1 gives a 16-to-1 configuration, a favourite circuit drawing question.
Exam tip
The examiner always asks you to implement a given Boolean function using a 74151 MUX — practice mapping the truth table output column directly onto D0–D7 inputs because that single step is where most marks are lost.