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PICList Thread
'RS232 Issues'
1999\04\05@060329 by Russell McMahon

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A "no chip" solution is potentially available.

This is more a hobbyist than professional solution but with care will
work very well indeed.

This does not meet formal RS232 spec and May not work with some PCs
BUT it has worked with every PC I have ever tried it with. I first
used it about 1980 :-)

It works on both data and control lines.

PIC --> PC

If you send "inverted" data from the PIC (ground = logic 1, +5volts =
logic 0) through 1 1K resistor to the PC RX data line the PC will
accept it as RS232 compatible. The 1K limits current in case you do
something wrong. A higher value may be able to be used depending on
port etc. .

PC --> PIC

You can also send data PC --> PIC in much the same way.

PC -------------/\/\/\/\----X-----/\/\/\/\------> PIC
                           R1                    R2

At point X place 2 diodes -

 A diode to supply (conducts if point X goes above supply - anode to
point X)  and
 A diode to ground (conducts if point X goes below ground, Cathode
to point X.)

R1 and R2 can be 10K or more (even 100K depending on speed). The
reason for 2 resistors is that point X will swing to 1 diode drop
above supply and one diode drop below ground (clamped by the
diodes). ]]

People will tell you you can get by with a single resistor and no
diodes !!!
Don't do it !!!!
Try if you must.  It will ***often***  work.
It relies on the PIC catch diodes working to clip the RS232's swing
above and below ground. This induces currents twixt pin and
This is bad practice.
I HAVE HAD IT FAIL - the PIC behaved in an ill/un defined manner.
This is the very worst type of fault of all.
Diodes can be 1N4148 but ideally be a schottky diode (eg BAT85). In
practice the 1N4148 is cheap and works well.

WARNING:  On some laptops especially,  the drivers make only about 3
volts positive AFAIR. This may not look like an especially good high
(5 volt) level to the PIC. An extra pullup may be needed - probably
a 1 Megohm from point X  to Vcc.

For higher data rates be sure that R1 and R2 do not slow the signal
too much when combined with the circuit's stray capacitance. Won't be
a problem with 10K resistors. I currently have a 9600 baud link
working with 2 x 100K resistors and a 1K transmit resistor. YMMV.


               Russell McMahon

From: Eric Oliver <spam_OUTericTakeThisOuTspamKEDCOENT.COM>

>My project currently uses a MAX232 to communicate with a PC.  I need
>extra line to the PC for some special handshaking so I searched
>site.  The MAX213E looks like what I need ( really more than I
need ) but,
>from reading the datasheet, it looks like the 213 only comes in
>mount packages.  So three questions arise.
>1) Does anyone know if this is true.  Is the 213 not available in
>2) How does one prototype with a SO chip?
>3) Any other suggestions for a one chip solution ?

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