William Chops Westfield wrote:
>     I can tell you, however, that the PC will receive data from the
>     PIC without one. Just remember you need to send the data
>     inverted, which means you can do it with a bit-banged interface
>     but not with the PIC's internal UART.
>
> A long time ago, after observing that the SDK-86 requiring +5 and +/-12
> used the +/-12V ONLY for the rs232 drivers, I connected the +12 supply to
> +5, and the -12V to GND, and it worked fine (talking to a CPM system,
> IIRC. (yes, that long ago))
>
> Since then, I've had a very "liberal" view of rs232 requirements. :-)
>
> There have been a couple of ciruits published (here?) that do clever things
> when at least one side has "real" rs232 voltages and "typical" rs232 signal
> leads.  Ie, rather than switch between +/-PWR, you can go between +5 and
> "whatever is on DTR (which is supposed to be -V, and I can damn well MAKE
> it be -V since I control the PC side software too.)"  (This is slightly
> more clever, IMHO, than using the modem signal leads to create a "power
> supply" for a traditional level converter circuit.)
>
> BillW

You can use a diode off RxD (perhaps other pins as well, coming from PC)
to an electrolytic's - side.  i.e. Cathode on PC side, anode to Cap's -
side, to generate -12V (or so.)  And same trick to generate +12v off the
same COM port.

Or, harder, run a charge pump voltage tripler off some PIC pins to make
+- 14V or so.

Schottky's can be a good idea here if you need more voltage (Seen
BAT85's sometimes used.)

http://uq.net.au/~zzdkeena/AVR/DCKProgSchem.GIF for one example, can
find others.

  Mark

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