> Some designs for RS232 interfacing:
>
> ; Simple RS232C - PC to PIC Interface Aug/94 T. Rudersdorfer,
> AScT
> ; revised Sep/94
> ;
> ; 1 - 470 ohm short circuit current limit
> ; 1 - 10k open collecter pull down
> ; 1 - 47k power up pull up
> ; 1 - 470k injection current limit
> ; 2 - 15pf oscillator stabilization
> ; 1 - .1uf decouple PIC and regulator
> ; 1 - 10uf voltage stabilization
> ; 1 - 5.0688MHz standard communication xtal
> ; 1 - 1N4150 reverse voltage blocking diode
> ; 1 - 78L05 5V low power regulator (100mA Max)
> ; 1 - 16C84 PIC
> ; 1 - LED do something test led
> ; 1 - 2k7 do something current limit for test led
> ;
> ; PC DTE DCE
> ;
> ; RX<(3)--(3)-/\/\/\/-----------------------------+--/\/\/\/----+
> ; 470 | 10k |
> ; |\ | | ///
> ; +-----/\/\/\/--| >|---------------------+ |
> ; | 2k7 |/ | --------------- | |
> ; | LED | 1 U RA1 |--+ |
> ; | ^ +5V | RA0 |----+
> ; | | | ____ 1 Osc1 |------------------+
> ; +---+-/\/\/\/--------| MCLR 6 Osc2 |------+ |
> ; 47k +-----| Vss C Vdd |--+ | _ |
> ; | +--| RB0 8 | | +---||_||---+
> ; /// | | 4 | | | 5.0688MHz |
> ; TX>(2)--(2)-/\/\/\/-----+ | | | _|_ _|_
> ; 470k |_______________| | ___15pf ___15pf
> ; | | |
> ; 1N4150 _____ ^ +5V | /// ///
> ; |\ | | | | |
> ; RTS>(4)--(4)-+----| >|----+---|78L05|---+------+
> ; | |/ | | 1|_____|3 |
> ; CTS<(5)--(5)-+ _|_ |2 _|_
> ; | ___10uf | ___.1uf
> ; DSR<(6)--(6)-+ | | |
> ; /// /// ///
> ; GND (7)--(7)-+
> ; |
> ; ///
> ;
> ; RS232 RX circuit background:
> ; Based on AN521 in the 1993 Embedded Handbook; PIC I/O pins can
> handle
> ; +/- 500uA of continuous injected current. This means voltages
> greater
> ; than VDD and VSS CAN BE used as long as adequate current limiting
> is
> ; provided. My original RX input design isolated the PIC by using an
> ; extra transistor, diode and resistor. This can be eliminated by
> using
> ; a large enough resistor value in series to the PIC I/O Pin. RS232
> ; voltage levels can be up to +/- 15V, therefore, a 470k resistor
> provides
> ; an adequate current limit of 32uA (the same current used in AN521).
> ;
> ; RS232 TX circuit background:
> ; Researching specification sheets for RS232 receivers like the 1489,
> ; reveals that they do not require bipolar voltages to operate. The
> ; 1489 in fact clips all -ve signals to ground, therefore only
> requiring
> ; +ve and ground input levels.
>
> Or have a look at the image at
>
http://www.ise.pw.edu.pl/~wzab/picadc/picadc.html
> ( on the middle of the page )
>