RE: [SI-LIST] : PCI length calculations

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From: Ingraham, Andrew (Andrew.Ingraham@compaq.com)
Date: Tue Feb 20 2001 - 10:43:06 PST


The PCI bus has no length specification.

> How do you arrive at the maximum length for a PCI 33 MHz bus?
 
Best done by simulations using your intended configuration (numbers of
loads, routing topology, trace impedance), and best-case and worst-case
device models.

The signal from any device on the bus must propagate to all other devices
and remain at a valid level within 10 ns (at 33 MHz for bussed signals, much
less for REQ# and GNT#), compared to the driver driving the test load.
Usually this means waiting for at least one reflection to return from the
end(s) of the bus. More detail can be found in the PCI electrical spec.

> I have done some calculations to arrive at the routed length required.
 
The presence of stubs and connectors tends to make simple calculations not
very useful.

> But the length is much lesser than the PCI length specified for 33MHz(
> which
> assumes say 12 inches as maximum)
 
Where did this come from?

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