RE: CPCI term resistors

["Ingraham, Andrew" <Andrew.Ingraham@compaq.com>]

> The CPCI specification requires 10 ohm series termination resistor on the
> PCI
> lines.  This minimize the effect of the stub on the backplane.  Standard
> PCI
> does not require termination resistors.  Why is this?
 
Standard PCI as a motherboard-only bus came first.  Then came the connectors
and PCI cards.  Standard PCI was designed without the resistors.

CPCI came later, and I believe their inclusion, along with a different card
connector, allowed more slots to be used with less difficulty.  At the time,
the typical bus had three, maybe four PCI slots, which was somewhat
inadequate for some cost-effective industrial applications.  CPCI allowed
eight without a bridge.

>   I am not sure the exact
> reasoning behind these termination resistors.
 
They somewhat isolate the stub from the rest of the bus, resulting in less
impedance discontinuity, or less capacitive load, to the main bus at each
stub.  There is some added delay "through" the resistor, which is weighed
against the reduced bus delay waiting for the bus to settle.

This is sometimes called distributed termination.

>   Lastly, what would be the effect
> of not having them  ;-).
 
I would expect you'd see worse ringing and longer settling times, but YMMV.

Andy