RE: PCI bussed signals trace impedance: Motherboard vs Expansion boar d

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

> The V/I curves included in the spec, I believe, are created based 22 ohm
> lines (The spec states they are created based on driving 10 equivalent
> loads then states that the calculations are done with 22 ohm lines?!?!?)
> is that to say that PCI spec ICs are capable of driving this impedance??
 

When a driver is in the middle of the bus, it effectively sees two bus
segments in parallel.

22 ohms is half of 44 which is less than the unloaded impedance because it
includes chips, stubs, connectors, vias, coupling, and so forth, all of
which reduce the minimum effective impedance.

Regards,
Andy