RE: [SI-LIST] : PCI bus termination Vs Pull ups

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From: Ingraham, Andrew (Andrew.Ingraham@compaq.com)
Date: Thu May 24 2001 - 06:26:15 PDT


> We are using pull-ups on the PCI bus. What is the effect of it on the
> termination ?.
 
Minimal, usually. Pull-up resistors are generally in the 2.5 - 10 Kohm
range, which is so far removed from the characteristic impedance that it
almost isn't there.

> How the refelected wave switching happens. Please clarify.
 
Quoting from section 4.1.2 in the PCI Spec, with my comments in [brackets]:

"This means that bus drivers are sized to only switch the bus half way to
the required high or low voltage. [Interpretation: Driver output impedance
roughly equals trace impedance. Voltage divider. Only half the
rail-to-rail voltage initially appears as a voltage step at the driver's
output pin.] The electrical wave propagates down the bus, reflects off the
unterminated end and back to the point of origin, thereby doubling the
initial voltage excursion to achieve the required voltage level. [The ends
are unterminated, open-circuit, reflection coefficient = +1. Causes
reflected wave to be nearly equal voltage magnitude to forward wave. Their
sum = twice magnitude of forward wave, so the step voltage doubles from the
reflected wave, approaching full rail-to-rail.]"

Andy

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