wrote:
Cleaning out the junk cupboard, I removed a graphics card from a HP craputer.
It is a fanless 1 GB GT620. I noticed that at least 10% of the edge contacts
are missing. So are there unused contacts (for PCI-E version 4 or whatever) or
is this a crippled card?

They can remove the excess ground contacts.

The ground contacts are there to try to maintain some
controlled impedance target level. Typically low amplitude
high speed signals have flyby 100 ohm termination at the end.
The actual terminator today is likely inside the chip on the receiver.

The grounds do a couple things. Shield one part from the adjacent
pair, on crosstalk. And are also part of the impedance (although
a tiny tiny part at that range). If could be that inner layers
have ground we can't see.

http://media.bestofmicro.com/B/D/989...e-slot-big.gif

The practice might have to be reversed when PCI Express Rev.4 cards
come out. Maybe the manufacturers will be "nervous" and put
them back :-) The manufacturers don't all necessarily have
big research facilities to be figuring this stuff out.
(You need a pretty expensive scope to be checking eye diagrams.)

Low amplitude high speed interconnect also tends to
use capacitive coupling. That makes it harder to
exceed common mode DC on the interface, or upset
the biasing on the receiver input pads. And that's how
you check whether a card is "x8 wiring" or not. You
count capacitors, the tiny ceramic caps coupling the
signals. At high frequency, even a small cap has plenty
of coupling (low Xc).

The GT710 is x8 wiring. This product pulls out the stops.
This card uses the shorter x8 connector. It removes the excess
ground contacts. And it has 8 pairs of caps for bus
lanes plus a pair of caps for the clock pair. It's
not a perfect specimen, as there are caps unrelated
to the business end of the thing in that picture.

http://www.palit.com/product/vga/pic...c15fd0008e.png

I could also find GT710 cards with the full x16 connector
and 16 pairs of caps and so on. But the video card
was "off-brand" and presumably this was part of
their marketing drive to snag consumers who thought
the extra window-dressing made a difference. Chopping
off half the wiring on a gutless card like this, makes
no difference at all. It could likely work just as
well with an x4 wiring plan. (Of course the marketing
department would kill the engineer for messing up
their marketing message if that happened. As soon as
consumers see the x4 wiring, they know it's a loser.)

(Example of a GT710 with all sixteen lanes. See picture of back
of card, to "count capacitors".)

https://www.storeeng.com/product/col...b-ddr3-64-bit/

Paul