RE: [SI-LIST] : PCI BUS: UNEQUAL LOADING

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From: Ingraham, Andrew (Andrew.Ingraham@compaq.com)
Date: Tue May 22 2001 - 09:59:04 PDT


I am not sure my mail is getting out today.... Haven't seen my earlier
reply yet.

> Lets consider a case in which the PCI bus length is
> long enough to create such effects. Let us
> also leave out the devices at the end of the PCI bus.
> Then I would expect the following effects:
> 1. The drivers would then be over-loaded because the
> loaded characteristic impedance could be
> lower than the impedance that it was designed to
> drive.
 
PCI was meant to drive that kind of a load too. The test load for 3.3V
drivers is a 25 ohm resistor (and a small capacitor). Why 25 ohms? It is
reasonably close to half the characteristic impedance, as a device would see
if it were driving into the middle of the bus (for a nanosecond or two).

> 2. There will be a mismatch between the driver's
> output impedance & the transmission line
> impedance, thus creating other transmission line
> problems.
 
Unfortunately, PCI is not "tight" enough that we can hope to achieve
impedance matching. Ideally, the driver's impedance is exactly half the
line's impedance when the driver is in the middle of the bus, or equal to
the line's impedance when it is on the end. But with driver source
impedances varying over a 5:1 range or so, even a close match to the line
(whose impedance also has a wide range) is wishful thinking.

That doesn't mean everything is lost, however. PCI still does tend to be
source terminated, to an extent, and then using the clamp diodes to help
control the overshoot that remains.

Regards,
Andy

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