Re: Q: 5V<->12V level conversion

--- In opendiag@yahoogroups.com, "Adam Bradley" <adam_j_bradley@y...>
wrote:
> Henrik,
>
> Firstly, I'm not ignoring you but your probably close to the mark
> about me not understanding and for that I do appologise. I am
> incredibly thankful for the input, suggestions and patience you and
> others have shown.
>

The thing is that it would probably save yourself a lot of time and
misunderstandings if you asked specific questions instead of tweaking
your design and hope that someone will spend time evaluating them.
Especially since you don't give any info about the reasoning behind
your design.

> Using the MAX232 was a suggested alternative from a different group
> (attached below). BTW, no RS232 line is connected directly to the
> AVR but rather are going through the MAX.
>

Oh yes you have, one of the RS232 outputs from the MAX232 is connected
to the AVR.

> My first search returned the following usenet post in
> sci.electronics.basics "Yes - one RS-232 transmitter should be able
> to drive a few receivers. However, you cannot have multiple
> transmitters talking on the same wire."
>

Exactly so.
[snip]
>
> ---snip---
>
> From: "Tom Deutschman" <autotraxeda@t...>
> Date: Wed Oct 22, 2003 10:13 am
> Subject: Re: Q: 5V<->12V level conversion (please check!)
>
> Hi Adam,
>
> You might want to consider using a Maxim (www.maxim-ic.com) RS232
> converter IC. Check out this link:
>
> http://para.maxim-ic.com/compare.asp?
> Fam=RS232&Tree=Interface&HP=Interface.cfm&l\n=
>
> As you'll see, they offer many different models, 3V and 5V supply, all
> will handle the 12VDC from the car. An added bonus is the ESD
> protection on the inputs. To use the transmitter, I assume you want to
> convert from 5V to 12V as well, put a diode in series with the output
> of the Maxim (or other RS232 transmitter) with the cathode pointing
> towards whatever 12V receiver you are communicating with.
>

Which isn't a very good idea unless you know enough about how the ECU
end is design. Probably not a big problem though. This is however not
the way your design was drawn.

> As far as converting from 12VDC to 5VDC, the RS232 receiver will
> handle this nicely. Doesn't matter that your input signal is between
> 0V and 12V. The RS232 receiver will handle this ok.
>

Now this I think is bad advice. Sure it should work in many cases, but
then you might end up using the RS232 receiver outside it's
specifications. The specification for RS232 defines that signal levels
between -3V and +3V are undefined, a RS232 compliant receiver may
treat them as either 0 or 1. In this case the MAX232 is designed in a
way that it will most probably interpret the input correctly, but it's
not proper design in my opinion.

/Henrik

 
Received on Sat Oct 25 07:37:39 2003