Re: Why does rs232 with no level conversion work?

The answer is simple Rick. thre are three kind of communications:
a. reliable
b. almost reliable in good defined conditions
c. not reliable

I'm really sorry you don't know romanian, I have an article on this theme.
However I'll try a very briefly explanation. The secret for a correct
interconection between two logic families (ie CMOS-5V microcontroller to
RS232) is to have enough noise margins on both high and low levels,
between those two families. With other words, for a functional RS232
communications you need more than +/- 3V at the receiver input
 ( +3V to +15V for logic 0 and -3V to -15V for logic 1) and for a long
distance between RS232 receiver and transmitter , minimum +/-5V at the
transmitter output.
Now, think a little, 0 to +5V at the PIC output is enough to create 0
logic for the RS232 receiver ( or 1 logic if you supply the PIC with
negative voltage), but it can't change the polarity of the
output tx logic level and generate the 1  ( or 0 in the mentioned way,
PIC with negative voltage supply).
 So, aparently the  tx will not work without having an opposite voltage
polarity than the PIC is supplied, used just for RS232 driver. The Rx is
much simple, the two diodes on any PIC pin will allow conversions from RS23=
2 tx to PIC
rx logic, with a simple resistor limiter.

But the Picaxe works. So, it can be easily included in b) or c) category
defined above. I do not know well the PICAXE even I have take a quick
look a while ago on it. See if it really use both rx and tx of the RS232
for communication. I suppose not.

top 10 wishes,
Vasile

On Thu, 1 Apr 2004, fun_4_me_now20012000 wrote:

> Hi Everyone.
>
> I always use some kind of line driver, but having read this post I
> decided to ask a question that has been bothering me for as long as I
> have been using uCs.
>
> Why can the basic stamp and the PICAXE use rs232 WITHOUT line drivers?
>
> A basic stamp or a PICAXE is just a pic with an "os" so,  what does
> it take to drive rs232 without a line driver with a pic?
>
> What are the limitations?
>
> Rick.
>
>
> --- In jallist@yahoogroups.com, Vasile Surducan  wrote:
> >
> >
> >
> > On Wed, 31 Mar 2004, swmillar wrote:
> >
> > > i haven't yet used JAL for serial communications but i'm using a
> > > 18F452, so i need a MAX232 chip
> > > for more than 9600bps?
> >
> >
> >   Max 232 is just for lever conversion from TTL/CMOS to RS232.
> > So, if you know other different methodes to convert, you don't need
> it.
> > BTW I've numbered 5 or 6 other methodes than using MAX232 chip.
> >
> > best,
> > Vasile
>
>
>
>