Hi, Adam,

This application is nearly what I was looking for, but I cannot see, how the
needed *negative* voltage for the Tx line is generated. In this application
the Tx pin of the PIC is simply connected to the Dsub plug resulting in a
voltage swing of just 0...+5V. But the spec says -5...+5V at least, optimal
is -12V....+12V.

Thanks anyway, may be this leads me further on.

Best regards

Peter


-----Urspr=FCngliche Nachricht-----
Von: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] Im Auftrag von
M. Adam Davis
Gesendet: Freitag, 16. November 2007 14:46
An: Microcontroller discussion list - Public.
Betreff: Re: [PIC]: RS232 to 5V interface without supply voltage

It's a pretty simple idea and interface, one example is here:
http://fly.cc.fer.hr/~mozgic/UIR/

Take one unused control line of the RS-232 port, put a regulator on it, and
use it for power.  Use the software on the RS-232 device to set the line to
a high (+12) state (which is usually low, space, 0 as far as the data is
concerned).

There isn't much current available per line (in theory there's a lot, in
practice there's little) so a lot of designs use diodes to combine more than
one line, and many designs use a low dropout regulator so they can still get
5V from a droopy serial port.

Also it's important to note that many new RS-232 devices only support 5V or
even 3.3V output (many laptops have this issue) which can in theory still
meet the RS-232 standard, but you may have more problems getting the power
you need form them.

For a one-off project, it's an excellent use.  For a production product, not
so much.  But then, a production product would probably fare better as a USB
device anyway.

-Adam

On 11/16/07, Peter Feucht <p.feucht@palmed-medizintechnik.de> wrote:
> Dear Piccers,
>
> I remember, that a long time ago I saw an application note for a
> RS232-to-PIC(5V) interface which worked without any external power =

> supply, but it took the needed voltages from some pins of the serial =

> port. It had just a RS232 9 pole Dsub and 3 Pins (Rx,Tx,GND) on the PIC
side.
>
> I just googled but wasn't successful.
>
> Does anybody remember such an application?
>
> Thanks for any help and best regards
>
> Peter
>
>
> --
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