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Triple Negative (Posted on 2004-02-12) Difficulty: 4 of 5
Using only two inverters and an unlimited number of AND and OR gates in a logic circuit, show how to invert an arbitrary number of inputs.

(For instance, if you have four inputs, the circuit will have four outputs that are the inverses of the four inputs)

No Solution Yet Submitted by DJ    
Rating: 4.1250 (8 votes)

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Hints/Tips It's Halloween so Boo..lean | Comment 16 of 19 |

Actually it can be done using only one inverter and OR gates.

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Notice that the problem does not specify if the input and output are series or parallel. I assume that both the input and output need to be parallel, otherwise the puzzle is simple. 

 <o:p></o:p>

The question is if converting to serial is allowed.  If so, it can be done with nothing more than or gates and one inverter.

 <o:p></o:p>

Aororor-                                 --ororor---not D

                           \                             /

Boror            \                         /     ------or--or--not C

                  \            \                     /     /

Cor      \           \                  /     /     --------or----not B

              \      \            \              /     /    /

D---------------------------not----------------------not A

 <o:p></o:p>

 <o:p></o:p>

I know the gates look weird, but both inputs of a gate can be tied to one input.  According to Boolean A + A=A.  The real problem here is the lack of a counter.  Although possible to do, it would be difficult to construct so that all the timing is correct.

 <o:p></o:p>

I doubt that this is what DJ is looking for, but it is a possible answer.  A comparator gate uses two inverters, but I dont see any way to attach numerous inputs to one comparator without going series. 

 <o:p></o:p>

I wonder if it is possible to do with a decoder or a MUX.  It might be, but I think this will also require an additional input to the two inverters, which is also not given in the puzzle.

It may be possible to also tie one of the given inputs into the inverters.  Looks like Im gonna have to open a couple of books.

 <o:p></o:p>

DJ, can you answer if additional inputs are allowed?
  Posted by Bruce Brantley on 2004-11-01 06:19:42
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