Displayport 1.8m cable works, but not 3m

[chris911t]

Setup:
Samsung S27A950D connected to an Alienware M18x laptop running dual nVidia GTX 580M.
The M18x has a mini-DP connector so I have to use a short mini-DP to DP cable so that I can then use a standard male DP to male DP cable.

Problem:
When I connect with a 1.8m cable it works fine. I have tried two different 3m cables though bought from different sources and in both cases the PC detects the monitor but only displays on the laptop screen.


Workarounds tried:
I have tried using only the Samsung, switching off the laptop display and it still doesn't work - it of a long shot anyway since the GPUs should have more than enough power.

Also tried various other ways of fiddling with the control panel screen res settings to try to activate the monitor.


Any suggestions? I haven't heard of any issues with longer cables causing loss of signal with DisplayPort - I heard HDMI has that issue in the past but I understood that it would degrade the picture quality rather than getting nothing at all.

Be grateful for any input.

 

[chris911t]

Anyone?
Or does anyone know of any issues with DP signal weakening through longer cables?

 

[wa1]

Umm, yes, longer cable is mean losing more power on it's way to the monitor.
Are you sure that 3m cable is fine (not broken)? Have a spare 3m cable? You can try other cable to make sure whether your cable is working or not...

 

[chris911t]

wa1 I'm not 100% the cables are ok but as I say I have bought 2 of them from different places and the same thing happens.

 

[wa1]

well, if the 1.8m cable is fine and 3m cable is not then I think you're stuck with that "rule", although I'm not even sure, 3m isn't too long. But consider that's a loss power, I have a laptop that has 3 usb ports, but when I use all of them (1 mouse, 1 modem and 1 charging hand-phone) then suddenly 1 port became not working, maybe this is also losing power problem too...

Anyone can help us?