First, it sounds like you're asking about overshoot, not undershoot. PCI
doesn't directly address ringback into the switching region. Assuming th=
you do mean "negative overshoot" where you wrote 'undershoot', then there
are parts to the answer.
1) PCI drivers, although absurdly strong for an unterminated environment,
*do* have their limits and you won't see 11 volts (or -5.5) even in a tot=
unloaded PCI bus driven from one end.
2) The receivers, on the other hand, should be overspecified and thus the=
are required to tolerate Thevenin sources of that magnitude. Note, _Thev=
sources. There is a lower bound to the impedance of the source, which le=
3) PCI nodes are also required to have overshoot clamps. Thus, a current=
Thevenin source of 11 volts won't actually take a PCI node to 11 volts bu=
a much more reasonable 6+ for a short time.
D. C. Sessions
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