PCI extension cable

[remzy]

Im planing on making PCI extension cable. I have all the components that I need for that project. I was only wondering about the wire size that I would need to use because I'm not sure how much current can a GPU draw from PCI slot on mobo.

[Adalhelm]

I believe the max power draw from PCIE is 75watts but that is just what I have read. Also why are you making your own and not just buying a pre-made cable?

[mariushm]

PCI Express or PCI (the old white slots) ?

 

PCI Express was 60w on 12v , 15w on 3.3v from the slot, but more recently the 75w on 12v is thrown around and some video cards pull up to 90-100w from the slot, for very short periods of time (they're supposed to pull on average below 75w)

Convetional PCI slots use 3.3v or 5v , up to 25 watts of power from the slot.

 

For 6pin or 8pin pci-e connectors, you're supposed to use AWG18 wire (or the equivalent in metric, which is about 0.75mm2 internal diameter of wires). Some power supplies use AWG16 wires, which are slightly thicker.

A single AWG18 wire is safe for up to 4-6 amps on short distances, so even a single pair of wires could deliver the current that's now on a 6 pin pci-e cable, but for redundancy and to lower the power losses in the cables themselves, 2 or 3 pairs of wires are used.

 

[SLAYR]

12 minutes ago, remzy said:

Im planing on making PCI extension cable. I have all the components that I need for that project. I was only wondering about the wire size that I would need to use because I'm not sure how much current can a GPU draw from PCI slot on mobo.

You can buy them for $5.

Look up pcie riser cable.

 

https://www.amazon.com/gp/aw/d/B00D79EV0G/ref=mp_s_a_1_2?ie=UTF8&qid=1473943316&sr=8-2&pi=SY200_QL40&keywords=pcie+riser+cable&dpPl=1&dpID=51zZEQZErlL&ref=plSrch

 

You will have to check pcie specification for the max voltages.

 

[remzy]

8 minutes ago, Adalhelm said:

I believe the max power draw from PCIE is 75watts but that is just what I have read. Also why are you making your own and not just buying a pre-made cable?

It has to be round for my application and not flat as all the others extensions.

[remzy]

6 minutes ago, mariushm said:

PCI Express or PCI (the old white slots) ?

 

PCI Express was 60w on 12v , 15w on 3.3v from the slot, but more recently the 75w on 12v is thrown around and some video cards pull up to 90-100w from the slot, for very short periods of time (they're supposed to pull on average below 75w)

Convetional PCI slots use 3.3v or 5v , up to 25 watts of power from the slot.

 

For 6pin or 8pin pci-e connectors, you're supposed to use AWG18 wire (or the equivalent in metric, which is about 0.75mm2 internal diameter of wires). Some power supplies use AWG16 wires, which are slightly thicker.

A single AWG18 wire is safe for up to 4-6 amps on short distances, so even a single pair of wires could deliver the current that's now on a 6 pin pci-e cable, but for redundancy and to lower the power losses in the cables themselves, 2 or 3 pairs of wires are used.

 

PCI Express

[SLAYR]

10 minutes ago, remzy said:

It has to be round for my application and not flat as all the others extensions.

You will have to solder 164 wires by hand then, and most of the wires will have to be exactly the same length because a small change may cause the riser to not work, if 1 out of the 164 wires that is just a bit longer then another it may not work, because it will take more time for the signal on that one wire to reach to slot.

 

It is the same situation with non buffered ram slots, they traces have to be exactly the same length, or it doesn't work.

 

EDIT: here is a pinout.

http://www.interfacebus.com/Design_PCI_Express_16x_PinOut.html

[remzy]

16 minutes ago, SLAYR said:

You will have to solder 164 wires by hand then, and most of the wires will have to be exactly the same length because a small change may cause the riser to not work, if 1 out of the 164 wires that is just a bit longer then another it may not work, because it will take more time for the signal on that one wire to reach to slot.

 

It is the same situation with non buffered ram slots, they traces have to be exactly the same length, or it doesn't work.

 

EDIT: here is a pinout.

http://www.interfacebus.com/Design_PCI_Express_16x_PinOut.html

Thanks for the pinout.

[mariushm]

If you don't need the bandwidth, you can just not connect some of the lanes, almost all x16 devices will happily run on x8 or x4 and switch to that automatically. You can just route a x4 or a x8 instead of x16 - each x1 gives you 500 MB/s. For video cards, between x16 and x8 there's very little differences, usually less than 1% drop in performance. Below x8, the performance drop will be significant enough to notice it.

 

See the pinout of the slot : https://en.wikipedia.org/wiki/PCI_Express#Pinout

 

If you plan to use round cables, you should use one with twisted pairs and ideally each pair shielded from the others, at least for the pci express differential pairs... for signal integrity in the slot (and therefore in the flat cable also), each differential pair is separated by a ground wire, if you use round cable the differential pairs will be too close together unless each differential pair is shielded from the others. Not saying it won't work without shielded pairs, just that there's a high chance you'll get transfer errors.

Or, you'd use one round cable with two wires and shielding for each set of differential pair + ground ... lots of thin (awg 28-30 would be enough) individual cables in a round bundle.

 

That's why you don't see lots of extenders with round cable, because it's more expensive and you still have to keep in mind that the length of each differential pair must be kept equal (can't have more than maybe a couple of mm of difference between differential pairs) so you'd also have to custom make the printed circuit board on which you solder the pci express slot to solder all the wires of the round cable in one place and then route traces to each pair but make wiggles in the traces so that the trace length going to the first pci-e differential pair would be same length as 4th or 8th differential pair.  It's really too much of a bother, flat ribbon cable makes things way way easier.