checking if two things do not equal None

Gregory Ewing greg.ewing at
Mon Mar 31 00:48:21 CEST 2014

Roy Smith wrote:
> Adding to the 
> confusion, many designs would use "active low" logic, which means a 1 
> was represented by a low voltage, and a 0 by a high voltage.  So, you 
> quickly end up with gibberish like, "not active low clear nand not 
> active low enable clock".

There are ways of dealing with that in schematic diagrams.
For exammple, if you have two active-low signals A and B
and want to express "A is active or B is active", you
draw an OR gate symbol with inversion circles on the
inputs. That's equivalent to a NAND gate, but makes the
intention clear.

Schematics drawn that way are much easier to follow than
ones that only use the inverted-output versions of the


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