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unused contacts on PCI-E 16x cards?
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? |
#2
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unused contacts on PCI-E 16x cards?
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 |
#3
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unused contacts on PCI-E 16x cards?
On Friday, December 20, 2019 at 11:55:23 PM UTC+8, Paul wrote:
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.) Could be for server mainboards that often have a heap of x4 slots for fibre channel, fast ethernet etc. And you want to put in a graphics card that is a little bit better than the onboard VGA. Although that would be a pretty small market. I can't think of an AMD x4 graphics card off the top of my head. |
#4
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unused contacts on PCI-E 16x cards?
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