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Two external displays on a 2014 iMac 5K

A

antibolo

macrumors 6502
Original poster
Sep 27, 2017
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Is it supposed to work? I'm trying to output to two DisplayPort monitors using the two Thunderbolt ports on my Late-2014 iMac 27 inch, but I can only get the first connected monitor to be seen, plugging a second one only gets it ignored.

The official documentation for this model mentions support for two simultaneous Thunderbolt displays. I'm guessing that it only works if they're specifically Thunderbolt monitors or something? That's a real bummer if true...

Does anyone here successfully use two DisplayPort (not Thunderbolt) monitors on this specific iMac model?
 
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Not enough information. 2 standard backlit LCD displays through el cheapo mini DVI–HDMI adapters are no problem. I've got a pair hooked up to a 2011 i5 right now.

For anything more exotic, it depends, Start here.

https://support.apple.com/en-us/HT206587

You can run a pair of 4K HDMI displays such as the $350 LGs on Amazon but you have to pay attention to the refresh rates per the article above.

BTW, used Apple TB displays are going for $150–$400 here in the silicon valley.
 
OK I figured out what's wrong.

Seems the 2014 iMac only supports two DisplayPort channels total. The thing is, my Dell UP2414Q uses MST for 4K at 60Hz, which means it eats up two DisplayPort channels, leaving none available for the second port. I tried shutting down MST support (which drops it to 30Hz, ewww) and then both displays work.

This is a real bummer but oh well. Seems there's not much else I can do.
 
FreakinEurekan said:
According to the tech specs https://support.apple.com/kb/sp707

"Simultaneously supports full native resolution on the built-in display and up to 3840 by 2160 pixels on an external display"

So, officially, just one display but it could be a 4k.
Click to expand...

While you're not wrong, the Apple support bulletin is more correct. Unfortunately, that results in

antibolo said:
I tried shutting down MST support (which drops it to 30Hz, ewww) and then both displays work.
Click to expand...

So, just how bad is a 4K monitor at 30Hz?

Enquiring minds want to know!
 
mikehalloran said:
So, just how bad is a 4K monitor at 30Hz?

Enquiring minds want to know!
Click to expand...

It's a waste of what I paid for, so no, I'm not going to downgrade to that.

Anyway, the second display I was trying to connect is actually a Luna Display dongle (https://lunadisplay.com) for use with Astropad. I'll just unplug my Dell whenever I want to use it. It's unfortunate but not that big of a deal.
 
eGPU such as your Luna does not work over OS 10.13.4–6 (unless over TB3, not an issue in your case) as Apple disabled that functionality. They re-enabled it in Mojave. Unfortunately, there was no 10.13.7 where they re-enabled it in High Sierra.

I don't know if your Luna is 10.14 compatible.
 
mikehalloran said:
eGPU such as your Luna does not work over OS 10.13.4–6 (unless over TB3, not an issue in your case) as Apple disabled that functionality. They re-enabled it in Mojave. Unfortunately, there was no 10.13.7 where they re-enabled it in High Sierra.

I don't know if your Luna is 10.14 compatible.
Click to expand...

The Luna is not a eGPU, it interacts as a physical display connected to DisplayPort. This has nothing to do with eGPU support.
 
I also have a late 2014 iMac 5k, have multiple 4k monitors and really wanted to have two hooked up. Having read the support documents and multiple forums I knew it wouldn't work but the displays are sitting here so what the hell.

Plugged both in and boom, both are at 3840X2160 @ 60Hz. Not sure why this works, since all supporting documentation says it won't. My monitors are Dell P2715Q running SST.
 
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Oh I'm sure that is why, it just goes against all the documentation that is provided by Apple. Just wanted to share my experience so my fellow 2014 iMac users can rock 3 displays.
 
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Macinsquatch said:
Oh I'm sure that is why, it just goes against all the documentation that is provided by Apple. Just wanted to share my experience so my fellow 2014 iMac users can rock 3 displays.
Click to expand...

Thank you for that information... I too am running an iMac 5K Late 2014 with one P2715Q 4K display attached. I very much need another 4K display, so I just ordered a P2721Q (P2715Q is no longer available) based on your findings (that the iMac will support two 4K displays running SST).
 
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Macinsquatch said:
Oh I'm sure that is why, it just goes against all the documentation that is provided by Apple. Just wanted to share my experience so my fellow 2014 iMac users can rock 3 displays.
Click to expand...
You're reading the wrong documentation. Last I checked, there are 6 interconnected pages. I had to keep clicking on the various links till they finally looped back to each other and I learned what I needed.

Anyway, Apple does provide the correct info but it's not easy to find.
 
One issue I've had with the multiple monitor setup is that the GPU kernel panics. Started to get progressively worse in the last 6 months. Now using an eGPU, no more crashing.
 
I got my new P2721Q today, it's running along with my old P2715Q on my iMac 5K Late 2014... all is well, both external monitors running fine in 4K mode. I'm very happy now. Running the new display using a Mini DisplayPort to USB-C cable. FYI, my iMac uses the Radeon R9 M295X GPU with 4 GB RAM.
 
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