Re: OT: Riddle for the bright minds here

From: sal (believer_at_nospam.org)
Date: 06/11/04 

Date: Fri, 11 Jun 2004 04:08:46 GMT

On Thu, 10 Jun 2004 23:04:43 +0000, Androcles wrote:

> |
> | Yes but that last line is important. You can build all the logical
> | operators, including 'not' and 'or', out of 'xor' and 'and'.
>
> You can build XOR out of NAND, which were the early and most common TTL
> gates, 4 to a chip. NAND lent itself readily with TTL. High voltage at the
> base is low voltage at the collector.
>
> | That isn't
> | true of 'or' and 'and', just because that last line is different.
>
> Sure you can, but that is wasting the gates and slows the system. Tie one
> input of a NAND gate to high...

But that's a NAND gate, which incorporates a NOT. As I said, you can't do
it with plain AND and OR, afaik. There's no way to make NOT with them.

XOR and AND => you can make the complete set

AND and NOT => you can make the complete set

OR and NOT => you can make the complete set

NAND alone => you can make the complete set

Just AND and OR => you need a separate NOT gate

But AND gates are kind of unphysical, aren't they? Real gates are
almost always NANDs, not ANDS. (Or has that changed with the advent of
whatever magic they use to drive chips at 2+ GHz? I'm pretty out of
date on the hardware end of things...) 

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