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
