OT: I2C pullup resistor location

[Jim]

I'm building a backplane board that will have a processor board (master)
and 8 slaves using I2C across the backplane. Is there any advantage to
placing the pullup resistors on the end of the backplane farthest from
the processor board? I recall installing active termination on the old
S-100 bus backplanes to overcome problems with ringing, I think. It's
been too long.

Thanks,
Jim.

[DJ Delorie]

I2C is driven from both "ends" of the bus, so if you want to use the
pull-ups as terminators, use two resistors, each twice the resistance,
one at each end.

I've never heard of anyone worrying about ringing on I2C though...

[John Doty]

Post by DJ Delorie
I2C is driven from both "ends" of the bus, so if you want to use the
pull-ups as terminators, use two resistors, each twice the resistance,
one at each end.

I2C pullups are generally a few kohms, far above any practical characteristic impedance, so they will have essentially no effect on ringing due to drive. They don't have enough drive capacity to produce ringing on upward transitions. On downward transitions the driver output resistance will be more important for damping.

Post by DJ Delorie
I've never heard of anyone worrying about ringing on I2C though...

I presume that I2C device makers choose drivers weak enough and receivers slow enough to avoid problems on reasonably sized boards.

John Doty Noqsi Aerospace, Ltd.
http://www.noqsi.com/
jpd-***@public.gmane.org