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Subject: Re: Archos <-> BMW IBUS
From: [IDC]Dragon (idc-dragon_at_gmx.de)
Date: 2004-03-02


(sorry for the previos mis-post, I hit the wrong button)

> As I understand from Jörg experiences, he connected the Archos directly to
> the MBUS and all went fine. I'm afraid that doing the same might damage
> either the archos or, even worse, a component from my car connected to the
> IBUS.

You won't damage anything in the car, worst case you prevent the bus from
working.

By connecting the Archos you prevent the bus from floating up to +12V,
instead this will be limited to about 4V (the 3.3V logic of the Archos plus the
forward voltage of its protection diode on the remote pin). You do "stess" (or
let's rather say, use) this diode. For a reasonable value of the pullup
resistors (kOhm range) this is no problem.
By limiting the bus voltage swing to 4V you reduce its immunity to noise. I
agree that this may not be the best idea if other components are on that bus,
too. But I'm sure BMW won't connect critical parts to the same bus that goes
to the radio. For me the direct connection was acceptable because it's
point-to-point, goes only to the radio. First I also thougt I need a level
shifter.

Have a look at this:
http://www.semiconductors.philips.com/acrobat/applicationnotes/AN97055.pdf

This is exactly what you need. The pullups are probably already there, so
all you need is a MOSFET and a 3.3V reference, which can be generated by a
simple resistor divider.

Jörg

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