Tony sez -

Is it worth dispensing with the 232 chips for PIC to serial port
connections?

I've seen countless simple interfaces that must work, or they wouldn't
be around.

I can see the RX working OK if it's inverted with a tranny, but does the
TX work after inversion when it can only be pulled down to 0 volts.


& Rusl sez -


Every time I see this question asked I jump in with the following advice and
end up starting a long thread and having various people tell me I don't know
what I'm talking about :-)

So, ignore this at your peril.
Your mileage may vary (but it probably won't).

DO NOT use a simple serial resistor to connect RS232 bipolar signals into a
PIC.
The PIC protection diodes will conduct and the results will be
indeterminate.
My practical experience shows that the processor may behave erratically in
quite unexpected ways.

Doing this violates the proper operating specs of the PIC if you read the
data sheets carefully. Lots of people can quote bits of the data sheet which
they say suggest its OK to do but they are the wrong bits - they relate to
absolute max conditions - not working specs.
Proper conditions on a pin are (from an obscure part of the data sheet which
I'm not going to look up) not more than Vcc (or Vdd or whatever) and not
less than ground.
Few people SHOULD be surprised by this.

What I do is either

1.    place 2 small Schottky diodes (BAT85 or friend) from the input pin to
Vcc and ground (connected to conduct only when the pin tries to go outside
supply rails).

or

2.    Use 2 series input resistors and place diodes as above (Schottky or
1N4148) to supply from the midpoint.

Method 1 also theoretically violates the data sheet max/min specs but the
Schottky clamp observes the spirit (not the letter) of the law and keeps the
pin voltages below the protection diode (significant) conduction points.

Method 2 is also not perfect theoretically but comes closer.

One could also use 2 x 5V1 zeners from pin to supply rails but the zener
knee is so soft that this is v v probably worse than Schottky.

Method 3:     Ignore me and listen to all the people who will tell you that
PIC protection diodes are just crying out to receive external currents and
sink them safely without detriment while the chip operates just as you'd
expect it to. Don't trust your life to the product though :-)
YM W V




      Russell McMahon
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-----Original Message-----

--
Best regards

Tony

http://www.picnpoke.com
mailto:sales@picnpoke.com