CarBytes - MAX232 circuit

[Steve Gorkowski]

You can't use transistors very easy because the edges of a rs232  signal must
have a slow rise time and fall time. If this part of the rs232 spec. is not
followed on long runs of cable  you can get cross talk on the rs232 lines and get
garbage on your receive line. In most cases I have found short runs (under 6ft )
work ok most of the time. Make life easy use a max232 ic .If you want my circuit
I will post it but none of these circuits are rocket science

Steve.

Steven

Steve

Peter Gargano wrote:

> Carsten's change to MAX232 circuit...
>
> Andrew K. Mattei wrote:
> >
> > >one question, What is the difference betwee Andrew Mattei's and Carsten
> > >Meyer's?  I planned on building Carsten's design because its more simple.
> > >What do the four transisters in the circuit?
> >
> > I'll field this one.
> >
> > Carsten's design is dependent on the characteristics of the diode in the
> [snip]
>
> In fact the latest incantation of Carstens interface uses a MAX232
> and a resistor and diode to combine the Tx and Rx ALDL TTL voltages.
> Carsten's original circuit had two transistors and a zener (diode),
> and I think that is was a problem, as the zener's voltage may have to be
> selected for the particular vehicle - anyway, he didn't think it was such
> a good idea (probably got too many emails asking why it didn't work!).
>
> So, are the transistors necessary (in Andrew's)? I suspect not (but I have
> not built either of these circuits) - I was just clarifying the confusion.
>
> This discussion also relates to my decoding of the '165 (and '808) circuitry.
> In the ECU, the Rx and Tx data is separated again, and goes to the SCI pins.
>
> PG.