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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