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

[Daniel J Wisehart <dw@yankee.us.com>]

On Friday 22 October 2004 07:11, Randall Nortman wrote:
> 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?

That is right: put the resistors in series with the signal.  They are even 
called series terminating resistors.  Typically you only want resistor when 
your bus is a line, but if you have a star type bus, you want one resistor 
per transmitter on the bus.

Daniel