Olin,

You are of course correct that there is no requirement
directly in the specification. From memory, I recall
implied from a diagram I was viewing at the time, that
when used in a long-distance (>10 meters) multidrop
configuration, i2c needed this "changing" resistor.

I don't have a direct reference to the diagram yet,
but I'm getting closer with page 20 figure 22A and 22B
at
http://www.philipslogic.com/support/appnotes/i2c/pdf/an255.pdf

Thanks for your clarification.

--- Olin Lathrop <olin_piclist@EMBEDINC.COM> wrote:
> > i2c, for instance, AFAICT
> > requires a resistor next to each node the value of
> > which is determined by the total number of nodes.
>
> There is no such requirement in the IIC spec.  It
> does define the maximum
> current any node must sink to drive a bus line low
> (3mA if I remember
> correctly).  IIC is usually implemented between
> parts on the same board.
> In that case you can usually ignore transmission
> line effects and provide
> a pullup at a single point.
>
>
>
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