# A Fool Needs Help

Discussion in 'Homework Help' started by DarkNovaBlade, Mar 23, 2010.

Mar 23, 2010
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I've been failing to keep up with labs, and now in a rush to do them all I have reached a lab which I have no idea how to do for my digital systems class.
The assignment is as such

(1)Design a logic circuit that uses a 74LS151 multiplexer to implement the Boolean function F(P, Q, R, S) = Σ(0, 1, 3, 4, 7, 11, 12, 13). The data sheet for the 74LS151 can be found at http://gauss.sdsmt.edu/shop/datasheets/ttl/151.pdf.
Note that you will need to connect the enable input, “E”, to 0 for the multiplexer to operate properly.
(2)Design a logic circuit that uses a 74LS151 multiplexer to implement the Boolean function F(P, Q, R, S) = Ʃ(0, 3, 4, 7, 8, 9, 13, 14).

It's mostly the using the multiplexer part that has me perplexed. I can design a logic circuit without using one for this boolean function, but don't really understand how to incorporate a multiplexer into such a circuit. any advice/help is largely appreciated, since I have more labs to do and need to finish by the end of today. Thanks in advance...

Last edited: Mar 23, 2010
2. ### Wendy Moderator

Mar 24, 2008
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A multiplexer can be thought of as a digital rotary switch. You can connect the input to either power supply rail, creating a 1 or a 0. The control lines (a digital number) will switch one of the input pins to the output, so you can create a truth table strictly by connecting the input to whatever logic level you need.

Many CMOS multiplexers act like true switches, in that the current can flow either way. This means they can be demultiplexers as well as multiplexers. TTL doesn't do this, the logic only flows one way.