Inverting signals was used for noise immunity. I believe Schmitt trigger
is apart of driver circuit itself too. 

John


> 
> On Sat, Nov 14, 2009 at 5:55 AM, Wouter van Ooijen <wouter@voti.nl>
> wrote:
> > In a lesson I want to explain asynch serial data
> format and RS232 signal
> > levels.
> >
> > AFAIK the RS232 signal levels predate the 5V TTL
> levels (correct)?
> >
> > The TTL-RS232 level converters/shifters are all
> inverting. Does anyone
> > know why? Because inverting drivers are slightly
> easier to build? Or
> > were the asynch-on-TTL levels chosen first, and as a
> consequence the
> > drivers had to be inverters?
> >
> > --
> >
> > Wouter van Ooijen
> >
> > -- -------------------------------------------
> > Van Ooijen Technische Informatica: www.voti.nl
> > consultancy, development, PICmicro products
> > docent Hogeschool van Utrecht: www.voti.nl/hvu
> >
> > --
> > http://www.piclist.com PIC/SX FAQ & list archive
> > View/change your membership options at
> > http://mailman.mit.edu/mailman/listinfo/piclist
> >
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