Hey Guys, Well I'm going insane over this comptuer architecture stuff. I studied the K-map and it's a breeze (I don't know why in the hell the proff made it seem so damn hard). Thanks to *anonproxy* for the K-map link. The books we use for referencing aren't very helpful either so I'm stuck searching for tutorials or asking fellow classmates for a little insight. However, my question lies in "don't care" terms. We're working on encoder and decoder circuits at the moment, and I need to understand how "don't care" terms actually work. I can't seem to find a good explanation with the time that I'm allowed to browse the web for sources or tutorials. Now I do understand it's literal meaning; a "don't care" term can't stand as a '0' or a '1' so it's marked as an 'x' on the map or truth table when constructing circuits. But I don't exactly understand how they are placed onto a map or truth table. If anyone can help me out I would greatly appreciate your time. Otherwise I'll keep searching for sources or go to the proff (which will probably be the end of my poor confused mind). - Rolos

The value of the dont cares are up to you.. you assign them a 1 or a 0 as you see fit so that you can minimalize the logic. Try switching the values of the dont cares around on one of your truth tables.. that should give you an example of how it can minimalize the logic design.

Dontcares (or "don't cares" for some) are a simple concept. You simply your truth table, like you might simplify an equation. This becomes very helpful with more inputs. This is a good example. http://www.shef.ac.uk/uni/academic/N-Q/phys/teaching/phy107/truth.html Take the rows of 0,0,1 and 1,1,1. A dontcare can easily replace those two rows with a single, more elegant, x,x,1 where x is either 1 or 0. You can see here what we did. We had two outcomes (actually four - we didn't include 1,0,1 or 0,1,1) and we represented them in one equation or row. So how do you simplify your tables? Take the top, unsimplified table in the link as your problem. Look for patterns (patterns lead to mathmatical relationships). Notice in the solution all simplified rows keep one element and make the other two dontcares. Once you see this it is really easy to catch on. Since your outputs are all one (aside from your first row), leave a 1 in every input position possible. Then replace all the other inputs with x. http://www.shef.ac.uk/uni/academic/N-Q/phys/teaching/phy107/phy107.html