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RE: PMC Spec - Characteristic Impedance Numbers?
Good point about PMC being based on the original PCI spec.
Pretty much lets us do what we need to do on our module.
I was concerned that a revised PMC spec might come out with
a fixed impedance value, but that would be closing the barn (lab?) door
after all the engineers designed PMC modules anywhere from 60 to 100 ohms!
At least the PCI traces on a PMC are typically very short.
From: Lame Brooks-G14738 [mailto:Brooks_Lame-G14738@email.mot.com]
Sent: Friday, October 26, 2001 11:14 AM
To: 'Gord Wait'; firstname.lastname@example.org
Subject: RE: PMC Spec - Characteristic Impedance Numbers?
Correct, the PMC spec (P1386.1) says nothing about impedance or trace
lenths, but does state that the electrical and logical layers are based on
the PCI Specification. Obviously, in some dense PMC designs, the PCI trace
length reqts might be dificult or unrealistic. The important part is that
the clock lenth be longer (~0.5") than the longest bus trace to ensure
settling time and the allowable impedance range is 60~100ohms; so, 65 is
fine with trace velocity of 150~190ps/inch.
Design, Validation, & Test Engineer;
Motorola Monterey Design Center
From: Gord Wait [mailto:Gord_Wait@spectrumsignal.com]
Sent: Thursday, 25 October, 2001 17:38
Subject: PMC Spec - Characteristic Impedance Numbers?
It's come up that the CPCI Spec requires 65 ohm impedance at the
backplane connectors (to match the backplane),
But we can't find a similar requirement for PMC modules. Anyone have
thoughts on this?
Is there a proposal to set an impedance target for PMC modules, or any
of the other PCI module standards?
(PCI cards are 62 ohms..)