Do "repeater" or "active" USB cables make a difference?

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WillPS
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Joined: Tue 22 Apr, 2008 18.32
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I've got a USB3 DisplayLink dock which I thought ingeniously I could create a KVM switch from by adding a manual USB3 switch.

It seems to work just fine with the desktop, but my work laptop doesn't seem to be able to sustain power/signal through the switch which causes the screens to loose connection at random intervals and it takes up to 20 seconds for them to reconnect. Most annoying.

I tried replacing the cables with better quality Belkin ones, which seems to have made a slight improvement.

I'm considering replacing the USB3 cable that connects the dock to the manual switch with a very short one too (it only really needs to be about 10cm).

Failing that, I've happened across these "repeater" or active cables on ebay. I'm skeptical about how much use they can be since they don't actually take any power of their own. Are they actually a thing worth having?
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Dr Lobster*
Posts: 2013
Joined: Sat 30 Aug, 2003 20.14

at work we've had the need to run usb cables over the 5m max length and we've used a variety of means to overcome the limitation.

those active usb cables do make a difference, but they are still flaky and keep dropping out.

we've also put a mini powered usb hub in the middle of usb two extension leads and had some success with that, but not always, but you can tinker, so on one side you have a 4m and the other a 2m cable etc to see what works best.

we once had some HP laser printers (think they were laserjet 2200D, so not bottom end) and they just would not work with any usb cable we tried over about 2m, even though the cables we had were good quality and within the 5m spec.

unfortunately, it's going to be a bit of trial and error.
cwathen
Posts: 1149
Joined: Fri 15 Aug, 2003 17.28

Active cables make a difference in that the chip helps to fix issues with attenuation and crosstalk as compared to passive cables, but I don't think you'll benefit in your case as their purpose is to achieve reliable connections on long cable runs rather to fix issues between devices connected on conventional 1M/2M passive cables - on these sorts of runs any cable should be fine.

Just to be clear on this, the displaylink dock works fine on all your equipment without the switch in place, it is only introducing the switch that causes the problem?

If it's just a cheaply made mechanical switch then it's just going to be contacts on a track switching between the two ports with no proper isolation and with QA testing being just for basic continuity. The basic design (even if extended for USB3) is likely about 15 years old and would have been intended originally for a usage scenario like connecting two computers to one printer, not something like switching displaylink.

Almost certainly, so much is happening to the signal going through the switch that the best cables in the world won't completely cure the problem.
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WillPS
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Thanks for the replies, appreciate the help!!
cwathen wrote:Active cables make a difference in that the chip helps to fix issues with attenuation and crosstalk as compared to passive cables, but I don't think you'll benefit in your case as their purpose is to achieve reliable connections on long cable runs rather to fix issues between devices connected on conventional 1M/2M passive cables - on these sorts of runs any cable should be fine.

Just to be clear on this, the displaylink dock works fine on all your equipment without the switch in place, it is only introducing the switch that causes the problem?

If it's just a cheaply made mechanical switch then it's just going to be contacts on a track switching between the two ports with no proper isolation and with QA testing being just for basic continuity. The basic design (even if extended for USB3) is likely about 15 years old and would have been intended originally for a usage scenario like connecting two computers to one printer, not something like switching displaylink.

Almost certainly, so much is happening to the signal going through the switch that the best cables in the world won't completely cure the problem.
There is some very brief drop outs when my laptop is connected (usually related to movement) but the signal recovers quickly enough that the screens come back within a couple of seconds (I think the problem comes when the dropout is long enough for Windows to register it as it then tries to rework the screen configuration). No such problem with the PC - this is what makes me think the problem might be the strength of the signal at the point of origination.

I accept your point though. Certainly the switch is a basic thing and the same seller on ebay sells similar things for USB2, VGA and pretty much every known connector.

Is there a more graceful way of using 2 computers with 1 dock, other than shoving a manual switch in place?
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