In spare time, I repair laptop computers. Pretty much all of the screens in them now use this "eDP" Display Port technology and I thought "this could turn out to be great for troubleshooting" because now there is some standardization. If a laptop came in with a blank screen for example, maybe I could connect a full size Display Port cable to the motherboard, to see if there is a fault with the eDP circuit or if the screen itself is dead. After soldering 5 or 6 jumper wires, I could potentially know where the fault is, in just a few minutes.
Microsoldering is no issue for me, and although this is a long winded way to test things (I could just go and buy an eDP tester or buy a known good screen) I did find it interesting to have a go at the cable – but I have had mixed results and need some help in determining what I'm missing.
My research found that eDP requires the following:
- Main Lane
- AUX Lane
- Hot Plug
…and ground lines.
I used a small 11 inch netbook motherboard that I know uses an eDP screen (I have the schematic "quanta_zhk_r3a_20140630_schematics.pdf"). I removed the eDP ribbon connector and soldered a terminal block with jumper wires to it. Then I connected the above lines to a standard Display Port cable, from which I had cut off the plug.
The external eDP cable was plugged into my Dell Display Port monitor, but it gave no image. I connected an HDMI monitor to the motherboard as well, so I could monitor what was going on (i.e. if a second screen was detected). I got as far as the motherboard detecting both monitors (they were both listed in Display Properties) but no matter what I did, I could not get any image on my external eDP screen. I tried all of the multi-monitor options and even booted without an HDMI monitor connected.
The Display Port monitor doesn't "wake" up at all, even though the computer is obviously able to read the displays information (which is apparently gathered via the AUX/Hotplug lines). So it would appear that these lines must be connected OK.
This motherboard only has one lane (lane 0). I tried connecting every lane (0-3) on the cable to lane 0 on the motherboard, but I still got no image and it would not wake up.
Could this be something to do with the motherboard only having a single lane? Or am I missing something?
I've got this far, I'd like to see it through! I have subsequently read of other people being successful in connecting, for example, iPad eDP screens to a PC etc. So why is this being so difficult?
Many thanks.
Best Answer
DisplayPort data lanes must be carefully made of "twinax" wires, to form a flawless differential transmission line. The DP data protocol uses "link training" to establish clock and symbol "lock" before being able to transmit data packets. Jumper wires presents (generally) horrible signal reflections, and if a link cannot find equalization parameters that provide certain error rate threshold, the link won't transmit any visual data.
So you need to fix the signal integrity on DP data lane.
ADDITION. Upon close examination of "quanta_zhk_r3a_20140630_schematics.pdf", the "eDP" port on this laptop appears to be a combination of 2-lane DP and 2-lane USB3 (to support touchscreen function). I don't believe that a standard 4-lane DP monitor can auto-detect and auto-negotiate the lane re-sizing, that's likely why this monitor shows nothing. This also could be a lack of firmware support on laptop side, since full DP was not an original option for this design.