>On Mon, 26 Jun 2000, Peter McAlpine wrote:
>
>> Hi All,
>> I have seen somewhere a circuit for how to connect standard
>> 5v output ports to RS232 serial port of a PC using a couple
>> of diodes... but  can't find it again.
>>
>> Anyone point me in the right direction please?
>
>I just connect the PIC's xmit pin directly to the PC RxD, and the PC's TxD
>to the PIC through a 22K resistor...  works fine for me.  You need to
>invert the data, of course.  I've also seen a lot of different methods of
>doing this with transistors, Zeners, etc. - I guess it just depends on how
>close to "real" RS232C you want to get, but the drivers & receivers I have
>looked at see anything below .7V as low and anything above 1.4V as high
>(if I remember correctly), so TTL/CMOS levels work fine.


*** Standard warning which always provokes bitter
*** does/does not/does too/ I do not/ Oh you Beast .... !!! argument :-)

Using a resistor alone to input over or under voltage data to a PIC pin will
SOMETIMES and non-repeatably and unreliably affect PIC operation.
Using the catch diodes like this does work often/sometimes but IS out of
spec and when it fails you can't blame Mchip.
It's happened to me :-)
It WILL happen to you sometime.
YMWontV.

Solutions:

- Reverse biased Schottky diodes to either supply from pin.
- 2 resistors in series for input resistor with 1N4148 type diodes at mid
point of resistors to supply and ground (back biased usually).
- More complex systems.

First 2 above actually technically still violate spec BUT the first prevents
the protection diodes conducting (which is the aim of the exercise and the
second may allow a minute amount of conduction of the protection diodes but
probably not.


RM