Hot answers tagged display
Yes, its called "closed clamshell" mode. In short, you'll need to attach your MBA to a mouse, a keyboard and power: https://support.apple.com/HT201834
You won't know until you actually get it and test it. HDMI specs say you should get 5v at 50mA. That's not much to start with. You can read about this more from here. You could get more power but it would have to have MHL (mobile high definition link) support. That will be able to give you up to 40 watts. Now, I looked on the Apple site and it ...
I had exactly the same problem, with the same monitor, Dell U2713 / U2713H / U2713HM / U2713HMt. I had two of them plugged in to my late-2014 Retina Macbook Pro 15" and only one was experiencing the problem. I tried 100s of different solutions (I can say for sure it's definitely not the cable!) and nothing worked. Just like you, I could get landscape ...
Yes, I think keeping the power on here is the key. When I remove the magsafe from my MBP it automatically goes to sleep and turns off the external display. If you really want to keep it from sleeping even when you are not connected to an external power source, you can use InsomniaX or Caffeine.
Do NOT use the HDMI video output! Please buy a Mini DisplayPort cable connect your MacBook and U2715H.
Copper-based Thunderbolt cables are limited to max length of 3 meters. Corning sells optical Thunderbolt cables you will need for the distance you specify here. You can get 10m lengths at Amazon for US$299. They come in 30m and 60m lengths for US$659 and US$1299, respectively. I haven't had the need for these lengths yet, but I did the research a few weeks ...
You can use the program ScreenRecycler to use another Mac (or even a PC) as a second monitor. http://www.screenrecycler.com/ScreenRecycler.html
Well, i guess you solved your problem by now, but I still wanted to answer, since I recently looked into eGPU options myself. If your sole aim is to run a 4K monitor (and your GPU doesn't support it), you could probably also get a USB graphics adapter. You find a selection here. This certainly is a much cheaper option, but probably too slow to watch ...
Based on my experience with Macbooks, I suspect there is either a loose connector, or (sigh) you have a dying chip on the motherboard (probably video controller). Genius Bar should be able to diagnose for you pretty quickly.
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