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What is the maximum length a PCI-Express bus can be extended with a PCI-Express Flexible Extension Cable like this (36 Pin):

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    What version of PCI express are you trying to extend? The environment you need to extend through makes a huge difference too, particularly with unshielded extenders. Mar 4, 2015 at 1:23
  • @DavidSchwartz In my case it is PCIe 3.0 but the other versions might be also interesting for other people looking here.
    – kimliv
    Mar 4, 2015 at 11:55
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    This appears to be a fly by night product operating outside of the pci express specifications. Going by the specs you need a signal retimer chip on each end and a much different looking cable that can be 1 meter long iirc. That is with copper of course. There are fiber solutions that can be much, much longer.
    – psusi
    Mar 5, 2015 at 16:04

4 Answers 4

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The maximum length for PCIe cable (all lane sizes) generation wise are as follows:

PCIe Gen 1: 15 inches.

PCIe Gen 2: 12 inches.

PCIe Gen 3: 8 inches.

Source

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  • Thanks, good to see someone experimentally determined what works in the absence of any official documentation. I wonder if there's any way a software tool could test and validate the integrity of the PCIe bus; I would really like to test my extension before I leave it in my desktop for good. Not so much worried about the quality, but rather that the length + existing PCIe lane length will be high enough that bus latency could decrease throughput even if the 8GHz clock rate is stable. After all PCIe lanes on a mobo aren't exactly laid out expecting this kind of aftermarket mod!
    – apraetor
    May 21, 2019 at 18:44
  • This is an incorrect answer. This depends on many factors and there is no official data on the cables because most systems have spec'ed for the port at a specific clock rate with a specific power usage and a specific tolerance for electrical noise at a specific trace length. Anything out of the tolerances for which the port was made for is indeterminate and varies wildly between systems. IE. The port is able to do PCIE 3.0 but no one designed it to have longer traces which means that maybe traces of 50 ft. might work and maybe a couple of microns longer on another machine won't. Nov 15, 2019 at 20:16
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the total valid length up to PCI-E 3.0 is reported to be 20 inch. it seems that 5 inch are "assigned" for usage by the PCB. So 15 inch are left over for extenders.

a relatively old research (might have been done on PCI-E 1.0; see https://www.overclock.net/forum/18082-builds-logs-case-mods/1427731-pci-express-extender-cables-benchmarked.html ) showed cable lengths in operation of 50, 30 and 20 cm to work quite nice - only stacking of two of the shorter cables to full length might have degraded the signal that much that a blue screen could be observed - the exact reason so far remains unclear so its open if alternatively the stacking rather caused a power supply issue instead of a signal quality issue.

according to this ( https://www.elektronik-kompendium.de/sites/com/0904051.htm ) source for PCI-E 4.0 the total length is reduced to 8 to 12 inch, leaving about 3 to 7 inch for cabling.

when checking with amazon for current offers you can find a bunch of "certified 4.0" riser cables (many of them are of the x16 type) with lengths of e.g. 30, 35 or even 45 cm - meaning the product offer is about 12, 14 or even nearly 18 inch. but even if you doubt this self-certificate or it wont work for your particular machine to PCI-E card model (with its internal PCB lengths) - the more general and also more trustworthy co-attribute of all those offers is that the given length will quite likely work for most PCI-E 3.0 setups because this means only half of the "certified" transfer rate is used.

relatively current publications (e.g. https://www.tomshardware.com/news/pcie-4.0-5.0-pci-sig-specification,38460.html ) on upcoming PCI 5.0 are just excluding the topic of raiser cards and cables all together, thus also staying quite silent about any length or even the pure continuation of this possibility.

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This looks like a cheap (unshielded riser) and the answer with the short lengths is probably true. On the other hand, watch this video https://www.youtube.com/watch?v=q5xvwPa3r7M

With good risers Linus achieved over 3m extension.

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    Linus achieves a lot of things. That doesn't mean it'll work with your particular motherboard and your other device. I wouldn't depend on it for anything where you need a reliable solution. Nov 17, 2018 at 10:47
  • Yes, that's usually true. But in this case he just used the more expensive Thermaltake PCIe Extender. Nothing else. No special modifications. Ofc it's probably outside the official specs and you have to read the reviews of this item. And ofc I don't see a real need to extend PCIe even more than 30cm ...
    – Mr X
    Nov 18, 2018 at 12:14
  • At what point does the length of the bus interfere? The longer the run, the longer it takes for the signal lanes to reach the correct signal state..? Shouldn't that mean it either cuts into the time between bits, or reduces the total throughput?
    – apraetor
    May 21, 2019 at 18:49
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One Stop Systems went a couple of hundred meters with their fiber optic cabled expansion chassis and daughter boards.

https://www.onestopsystems.com/product/pcie-x4-fiber-optic-cable https://www.onestopsystems.com/product/nanocube-expansion-enclosure-one-pcie-x8-30-slot

But for a small cable like that, it is really determined by the motherboard and your motherboard's north bridge's ability to handle electrical noise introduced by having physically longer traces. Shorter traces are generally better and some cables, much like the one you have shown, have introduced shielding to help prevent noise from being introduced on the longer traces but ultimately it's your motherboard's tolerance for that noise which will determine how long you can make a cable as you desire. This will vary wildly between different motherboards, even between different motherboards of the same make and model. For example: I used to test PCI-E charactaristics for a fortune 500 company's NVME drives. As a group, we would buy dozens of motherboards and then cherry pick a couple which had the best noise tolerance for performance testing. Although we never attached any extension cables to these boards, I imagine that you could get some pretty long cables to work with those boards because of the amount of electrical noise that they were able to tolerate. We would then take the boards which didn't meet those stringent requirements and then ship them to other parts of the company for use.

Your results will vary wildly with the use of such a basic cable. Maybe 50 Ft. Maybe 1 inch. This is not really determined by the cable.

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