Re: gEDA-user: General ground/power plane design questions

[Randall Nortman <geda-lists@wonderclown.org>]

On Wed, Oct 20, 2004 at 07:26:18AM -0700, Daniel Wisehart wrote:
[...]
> > 2) I would like to be able to extend the SPI and I2C buses between
> >    stacked boards for short distances (perhaps three boards stacked
> >    1.5" apart with about 4" total cable length).
> 
> We use 49.9 ohm terminating resistors at the source end to take care
> of these reflections.  When there is bi-directional traffic, the
> resistors are only needed on one end.

Do you mean that you put these in series, rather than pulling up to
supply or down to ground?  Also, because of the stacking of boards, my
buses will look more like a fork than a line -- each board will be a
prong on the bus of ~12cm, there will be a ribbon cable of ~10cm
connecting them all together along one end (more like a parallel
connection than series, with one end of the bus on each board left
unconnected).  Should I perhaps put a terminator on each board?  At
the unconnected end or at the inter-board connector?