From: [IDC]Dragon <idc-dragon_at_gmx.de>
Date: Fri, 12 Mar 2004 11:30:32 +0100 (MET)

> debug the trasmit routines. So, I can send and receive packets to the bus
> with the archos with this setup:
>
> PC <-> RS232-IBUS adaptor <-> IBUS wire <-> MOSFET Level Shifter (3V-12V)
> <-> Archos
>
> The connections on the MOSFET are:
>
> Source - Archos Remote Control Pin
> Gate - 3V Power Suppy
> Drain - BMW IBUS

Looks OK to me (you got that from the application note I pointed to,
right?).

> The trouble is that when I connect the archos to the car bus with the
> mosfet
> level shifter (I used a BS170) when the archos forces the bus low it can
> only bring it to around 1.2 Volts and when I look to the drain pin I still
> see around 12V. If I short circuit the ground with the source pin then I
> can bring the drain pin (connected to the car BUS) to around 0V.

I have used the same transistor when I started my hookup with a level
shifter. It worked, and later I found it's not needed.

The transistor is very sensitive, but fortunately very cheap as well. I blew
one or two in my experiments, half-damaged MOSFETs gives strange effects
which may explain what you see.

> I really don't know how to solve this and it would be great if you could
> help me. Would it help connecting to a different MOSFET with a lower Vgs
> threshold value?

This part should be OK.
Another explaination could be that the pullup is very strong, so the Archos
fails to pull it low. How much current do you measure when you short source
to ground (or bus to ground)? The SH is specified to sink max. 10 mA.

Jörg

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Received on 2004-03-12