Chris (Carr): do you mean this:

 The I 2 C-bus and how to use it
 (including specifications)
 1995 update

? Cos' if you do, this is the document I referred to in my original
posting. I will be using Rs as in Fig.24 in said document but still I'd
like to solve the mystery. In the end I'll have to tristate I think, or
use two pins on the host for SDA and a NPN as OC driver, or even a shottky
diode and a resistor in antiparallel. Oh well, I tried hard.

thanks to all who answered, esp. Roman Black, Olin Lathorp and Chris Carr.

Peter

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