17 Jun
2011
17 Jun
'11
3:55 p.m.
On 17 Jun 2011 at 15:13, dwight elvey wrote:
The only problem with that is that in some cases, it
might be that one
input doesn't match the logical sense of the other inputs. Now that
I've seen this in real circuits and not someones RTL like diagram, I
realize that I'd rather just see a nand gate look like a nand gate.
I've found that I can't trust the demorganed symbol tell me what any
upstream signal really has on that gate. I have to back trace it,
anyway, for all but the simplest circuits. Dwight
If I see a two input OR with bubbles on both inputs and the output, I
automatically (i.e. without a lot of conscious thought) say, "That's
a negative-logic OR that's masquerading as a positive-logic AND gate.
It's pretty much automatic, like resistor color code--you don't
mentally run the color chart or some inande mnemonic aid in your
head, you just grab a 47K resistor out of the pile because it looks
like a 47K resistor.
So give me the DeMorganed representation any day. I'll have more
confidence that the designer has his thought processes straight.
On the other hand, conventional logic symbols are weak in that they
shows the component units, but doesn't necessarily show what they're
supposed to do. For example, it may be expeditious to construct a
XOR gate from leftover NAND gates and inverters, but there's no good
way to show the intended function and still keep the components
identified.
--Chuck