CarBytes - MAX232 circuit

[Peter Gargano]

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.