# Truth table and logic circuit for 4x1 Multiplexer

#### KevinEamon

Joined Apr 9, 2017
284
Hi guys;

I think my lecturer is being a bit naughty.
Studying for my first tests here... He's given us a piece of VHDL and asked us to do the Truth Table and logic circuit for it.
The answer he's given is basically what looks like a flip flop waveform pattern output.... Hmmm...Well that's not a TT or a logic circuit right?

The logic is -
Y<= (not A0 and not A1 and D0) or
(A0 and not A1 and D1) or
(not A0 and A1 and D2) or
(A0 and A1 and D3);

Now I was having a think about this. Looks easy, but it's tricky... After a number of attempts at wires crossing all over the place and 32 bit truth tables etc...
I think this might be right... Maybe??? What do you think?

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#### dl324

Joined Mar 30, 2015
14,302
What do you think?
Your circuit makes no sense. You are shorting some of the inputs and you have gates with a single input from the data lines and wire OR'ing them to an output gate that looks like a single input OR.

I don't think you're going to be able to generate a Kmap by taking short cuts.

#### KevinEamon

Joined Apr 9, 2017
284
Well I was thinking about that. In another construction I had 3 channels for the OR gate. But if you think about it - they all then output as Y, right? So is it a shortcut? Or is it perhaps - elegants manifest?

#### Papabravo

Joined Feb 24, 2006
18,067
Your circuit makes no sense. You are shorting some of the inputs and you have gates with a single input from the data lines and wire OR'ing them to an output gate that looks like a single input OR.

I don't think you're going to be able to generate a Kmap by taking short cuts.
That is the STANDARD shorthand used by PAL devices.
The dots represent intact fuses. The crossed wires without a dot represent "blown" fuses.
The 4 gates on the right are multi-input AND gates with unused inputs tied high.
The single gate in the lower right is a multi-input OR gate with unused inputs tied low.
The result should be an easily decipherable truth table for a multiplexer in SOP form.
Bravo to the TS for figuring out as much as he did.

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