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Subject: Re: BMW IBUS <-> Archos Jukebox (as CD Changer)
From: [IDC]Dragon (idc-dragon_at_gmx.de)
Date: 2004-03-17


> I remade the circuit with just one transistor and it is working well as
> long
> as the pullup resistor on the (simulated) bus is at least 10K. For 5K the
> archos fails to put the bus low but it still can receive. It is definetely
a

This sounds like your box is kind of weak.

> limitation on the current that the archos can sink per pin. At 1.2V (which
> is what I measure) the current is 8mA (by the SH spec). Now, with a pullup
> in the car bus clearly around 1K (or even lower, since the specification
> says that each node in the bus is required to sink at least 40mA and a
> maximum of 100mA should be possible in the worst case) what can I do to?

This is definitely beyond what the chip can sink.
You'll need something like a trandceiver/booster for an I2C bus. They do the
same on the data line, be bidirectional by pulling it low at either side.
I'm sure there are existing solutions.
The "challange" of such a booster is to detect who's pulling low. It must
only pull itself if it was our side, not for the other.

> Can
> something (transistor?) be placed at the archos pin that turns on when the
> pin gets low and whose primary function is to divert the current that is
> coming from the IBUS and sink it to the ground? I guess what I needed
> would
> be something that would sink 80% of the current that is coming from the
> drain to the source of the MOSFET avoiding it to enter the archos pin...

Not so simple. I once made a circuit with two antiparallel diodes as the bus
connection, to detect which side is lower and therefore the driving one.
There must be a chip to do this standard job (in I2C world), I just don't
know one free-handed.

> If this is stupid or unrealizable then I guess I will have to make the
> serial mod and use some real IBUS transceiver like the Melexis 3122
> chip...

Less elegant, requires a modded box.

Jörg

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