Friday, September 18, 2009 11:36 PM,

No sooner do I send the previous message than I receive another from a group 
that is struggling with 6b.  They are trying to be creative, so I thought I 
would forward their question and my response.

But before that, I notice that the mailer distorted my last email on 6b by 
distorting the "NOT" symbols (the bars) above some of the logic variables on 
the example I constructed.  Please simply ignore the logic equation and use 
the truth table as the function I wish to implement using an 8to1 MUX.

Now then, from a student:



	Professor Patt:

	I'm having a movie night which digressed into a "lets all 
	try to solve this seemingly impossible (actually probably impossible)
	question" concerning 6b.. Coincidentally, all of the people I was 
	watching a movie with happen to be in my study group.

	The question asks: "Implement F =3D a XOR b using ONLY two 2-1 muxes. 
	You are not allowed to use a not gate (not A and not B are not 
	allowed)."

	We have worked on this problem for several hours and have concluded 
	that if all of your inputs into a mux are 1 then there is no way to 
	get a 0.

	However, we came up with a slightly creative way to rig the muxes 
	together and were not sure if its legal. We tied the output of an 
	and gate from the previous mux to the input of the and of the next 
	mux. We are not sure if this is legal because when you draw a mux in 
	its trapezoidal form there's no way to make internal connections like 
	that; however, if you were setting up the gates inside of a computer 
	it seems like it would work.

	<<name withheld to protect the student who is creative but wrong>>



Please pass this on to your study group whose members are studying and 
watching a movie at the same time.  Hope the movie is boring so you can spend 
more time on the problem set :-)

Actually, it can be done, but no, you can not pull a wire from inside the MUX 
and attach it outside the MUX.

BUT the problem really is a trick question - which I designed especially for 
you.

So, although it will never be on one of my exams, I thought some of you would 
have fun with it.

Good luck with the rest of the problem set.

Yale Patt