iButton io protection

Archiver

Hello Stampers


Hi guys, it has been a while but I'm back in Stamp business :-))

I hooked up a Dallas iButton probe DS9092 (
http://dbserv.maxim-ic.com/quick_view2.cfm?qv_pk=2974 ) to the io pin of a
BS2P40 used in an entrance control circuit.

The 4.7k pullup resistor is in place and I can read the code embedded in the
serial number iButton DS1990A (
http://dbserv.maxim-ic.com/quick_view2.cfm?qv_pk=2829 )

I hear you thinking, so what the heck is the problem, well, because this
system will be used in a 12 volt car environment, I want to protect the io
pin against a lot of situations:

- shortning the reader (the input will be pulled down to GND) while the
OWOUT command sends data to the iButton
- putting 12 volt (or any other voltage) between the outer and inner "ring"
of the reader (a iButton like battery or similar, a miswiring etc.)

I already tried a 1k resistor between the io pin and the pullup resistor but
then I can't read the iButton anymore.

Does anybody knows a good protection circuit (I already toasted an io port
on the BS2P40) so the iButton reader is a bit vanalism proof.

Thanks in advance

Michel


Met vriendelijke groetjes,

> ************************************************
> Michel De Meester
> Biotechnisch Onderhoud
> Universitair Ziekenhuis Antwerpen
> Wilrijkstraat 10
> 2650 Edegem - B
> tel: ++32 (0)3 821 36 47
> e-mail: michel.de.meester@u...
>
> *************************************************
>
>