Re: [SI-LIST] : Preferred PWB impedances
Andrew Ingraham (ingraham@wrksys.ENET.dec.com)
Thu, 20 Nov 97 07:49:07 EST
> > > > b) PCI
> > > > If you stick to the spec, you have to use a 30-ohm
> > > > reflected wave driver, and lots of power/ground pairs.
> > Intel calls PCI a reflected-wave switching environment, but
> > it's really just the same old TTL-type signalling with some
> > limits to keep the worst violations from getting out of hand.
> > It's a cut-to-fit job instead of a designed-from-first-principles
> > high-speed bus. That's OK, but we shouldn't kid ourselves.
> I'd really love to read the minutes of the original PCI meetings.
> I am still quite impressed with the electrical part of the
> spec. Especially how it takes into account component
> manufacturers resistance to change their electrical specs!!
Very early versions of PCI, before expansion cards were introduced,
had two driver types. Class-I was an incident-wave driver. Class-II
The Class-I driver (dropped around version 1.1 of the spec) apparently
was intended for discrete implementations. The faster bus switch time
allowed several nanoseconds more driver delay time.
The drive strength of Class-I drivers apparently was 149mA at 0.5V.