From: Fernando Birra <fpb_at_di.fct.unl.pt>
Date: Fri, 12 Mar 2004 13:05:37 -0000

Hi again,

"[IDC]Dragon" <idc-dragon_at_gmx.de> escreveu na mensagem
news:18111.1079087432_at_www6.gmx.net...
>> 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?).
>

Yes, that's 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.
>

In my case if I connect the archos directly to the bus I read around 8V at
the pin/bus when the archos forces the bus to low state.

>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 tried changing and it still did't work. Also, doesn't the fact that the
direct connection (without any level shifter) also has the problem of not
being capable of driving the bus low suggests that the problem are not the
transistors?

>> 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.
>

I'm not sure if I understood it (I really know very little about
electronics). Do you want me to measure the current that through to the
source or drain pins when either the source or bus is short to ground? I
tried but I always got 0 mA, I'm sure I made some mistake...
Can I connect one test lead of the multimeter to the ground and the other
one to the source pin to measure it?

>Jörg
>

Thans for all your help.

Fernando Birra


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Received on 2004-03-12