AVR-Chat

On Fri, 2005-09-02 at 10:33 +1000, John Samperi wrote:
> At 12:47 AM 2/09/2005, you wrote:
> >Witness RS-232-C and IBM,for example.
> ...................
> >shoudl be, until I realzied I needed to change the sense of my logic.
> >Switched to having ground potential read as a logic one,and Vcc as a
> >logic zero, and everything ran perfectly.
> Do you mean that you didn't have an RS232 chip connect to the
> AVR? =-O

   Why would I have had an RS-232 intercae connected?  I was reading a
bank of pushbutton switches.

> EVERYBODY :-) knows that the RS232 standard uses -12V for a one
> and +12V for a zero ;-) but if you do the right thing and use
> the correct interface then 0V is logic 0 again in your system
> and don't have to worry what happens outside. Many people make
> the mistake of hooking up the RS232 of a computer directly to
> the micro's UART pins :-(

 Hooking up a 13 volt bipolar communications line to a 5 volt general
I/O pin on a microcontroller would be downright stupid to begin with.

  But thanks for pointng out, and saving me having to, that there is a
erfect example of where a positive voltage is *NOT* a one in  standard
computing.  I defy ANYONE to show me where it is codified that a
positive voltage is a logical ONE anywhere in the literature.  That is a
device specific issue.

Tom