Felix Collins/PEC/NZ wrote:
>
> Hi,
> I'm very new to the PICnic and have a question that may be a bit simple for
 the
> group.  However I can't find anything in the FAQ's.
>
> The problem is that the transmitter seems to be inverting all the signals.  It
> seems to idle high, but the start bit, data and stop bits are all in the wrong
> polarity (start bit is high and stop bit is low).  The receiver also seems to
> be working incorrectly as it doesn't correctly receive data being sent.
>
> I can't find anything in the documentation about polarity inversion or
> anything.  Am I just missing something really obvious?  This application is
> just for inter-processor comms between a 3v DSP and a 16C63 pic.  I'm running
> it at 312.5 kiloBaud, 1 start, 8 data, no parity, 1 stop.
>
> Thanks for any suggestions,
> Felix

This reply will be one of many, I guess, but here goes...

The TTL level from you chip needs to be converter to the 12V/-12V RS232
levels.
You'll notice on the symbols of all the RS232 driver chips (MAX232 et
al), that
the drivers are indicated as inverters. Thus 5V <-> -12V and 0V <-> 12V.
If you
are in the development phase though, you can get away with the following
on a
PC or laptop :
                  5V
                  ---
                   |
                   \
                   /
                   \  10k
                   /
                   \
                   /                      Out
                   --------------------------
                   |
                   | Collector
  In     10k    | /
  ------/\/\/---|    (NPN)
                | \
                   | Emittor
                   |
  -------------------------------------------

Use this simple inverter to change the levels. It has never
failed me. I'm sure a whole bunch of complicated circuits will
be posted, but this one also works.

--
Friendly Regards

Tjaart van der Walt
mailto:tjaart@wasp.co.za
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