RE: CPCI term resistors

[tlyungha@collins.rockwell.com]

>  CPCI is meant to be used for the backplane and the lengths are extremely
> large. reflected wave switching cannot be used and needs the terminations
> on
> the bus.

The ideal case is for the signal to reflect off of the end of the bus and
return, with any additional reflections or distortions not affecting the
state of the signal at any of the cards.  Stubs along the bus will create
small additional reflections  which could distort the waveform in such a
way that all cards do not see the signal stable in the required time
(10nsec on a 33Mhz bus).  I believe that the stub resistors, by placing
them close to the connector, partly isolates the bus from the stub (high
frequency isolation), improving the signal quality . These 10 ohm resistors
were not intended to serve as a series terminator when the card drives the
bus (series termination is generally only effective on a point-to-point
connection, the resistor should be near the driver not the connector, and
10 ohms is not a very good match for a 50-60 ohm backplane anyway).

In addition to the requirement for series stub resistors on the board, the
CPCI specification also defines the use of schottky diode termination on
the backplane (at least for an 8 slot backplane).  The specification claims
that with a lightly loaded 8 slot backplane (e.g. only a few cards
populated), the signals do not settle in the required 10nsec, and these
diodes will solve that problem.  The intent of the diode terminators is to
shunt any overshoots or undershoots, to minimize the number of reflections
required to stabilize the bus.

In theory, the CPCI people have done simulation and testing with these
approaches.  If you do not comply, you are on your own.

Tom