Once upon a time Phil Eisermann said:
> > piclist-bounces@mit.edu wrote:
> > I need a simple logic function, a two input AND feeding a two input OR
> > feeding another two input AND [see picture]. Since I don't need any
> > other logic in the project I hate waste the space and use two chips,
> > is anyone else's Boolean algebra less rusty that mine? Is there a one
> > part implementation (other than a PAL or CPLD)?
> > 
> 
> umm. a PIC <vbg> 
> 
> seriously, though. I don't see anything simpler than D[AB+C] e.g. the 
> circuit you have shown. The only 'trick' i remember off the top of my
> head would be the old 'decoder as a minterm generator', but that was
> already old way back when I was young :) Do they still teach that in 
> school? In any case it makes no sense in this application.
> 
> Unless you can 'design-out' the need for the logic function?

When I first read this post I knew there was another way to do this, but I 
couldn't remember what it was.  I was just sitting here thinking about 
other things, and it came to me.  You need three 3-Input NAND gates, such 
as found in a 74HC10 or equivalent.

Wire A, B and D as the inputs to the first NAND gate, giving !(ABD) as the 
output.  Wire C and D to the second NAND gate, tying the third input high, 
gives !(CD) as the output.  Lastly, you wire the outputs of the first two 
NAND gates to inputs of the third, again tying the unused input high.  The 
output from the third gate is now !( !(ABD)  !(CD) ).  Using DeMorgan's 
theorem, this simplifies to ABD + CD, which is just another way of writing 
D(AB+C).

I hope this is of some help.

Paul VanGraafeiland
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