2017 iMac external monitor problems

[iMacDonald]

What's the deal with this?
There are only a few monitors that have the native USB-C connection from what I can see, and not exactly cheap.

I'm still using one of my Dell 2715H after selling the other, quite a popular 27 inch monitor along with my new 2017 iMac.

I'm on my THIRD USB-C to displayport cable.
All different brands.

So far cables from Plugable and CHOETECH have both initially worked but then when they get warm after a while, start to fail. The picture breaks, starts to erratically flicker and disappears altogether or can cause weird power issues where the monitor does not even switch off anymore.

The third one I'm on is the cheapest cable from Rankie, which I noticed another user in the reviews already said failed after a while with his iMac Pro. Probably just a matter of time before that fails for me too I guess.

Wish the iMacs still had thunderbolt 2 / mini displayports. Those seemed a lot more reliable / better compatible with third party screens and third party cables.

So is the solution forking out $50/£50 for the ONE Moshi USB-C to displayport cable that is on the Apple store or just buy a USB-C monitor?

Other than this malarky, the new iMac I bought seems great.

 

[iMacDonald]

The 3rd cable just failed. I would definitely avoid the Rankie cable. Eventhough it worked the longest I had even trouble removing that one from the USB-C port. The iMac even froze whilst doing that!
Screw this USB-C.

 

[theluggage]

I've been using the Plugable USB-C to DisplayPort cable to drive a Dell S2817Q for 6+ months with no problems. Maybe you were unlucky?

Wish the iMacs still had thunderbolt 2 / mini displayports. Those seemed a lot more reliable / better compatible with third party screens and third party cables.

Hear, hear!

It's a desktop - plenty of space for a couple of Mini-DP sockets. Shoe-horning multiple functions onto one connector is an unnecessary complication that's only justifiable on tablets and ultra-portables - and unlike Thunderbolt 1/2, the USB-C "alt modes" clearly need more than a simple, physical adaptor to drive DisplayPort devices. Apple couldn't even contrive to have a suitable adapter in their store (...at least not in the UK store, the Moshi cable on the US store looks like a new arrival...)

NB: Irrelevant to your problem, but that Moshi cable is intriguing - 5k support over a single USB-C cable would require DisplayPort 1.4 which I didn't think was supported at all by current Macs, thanks to the "Alpine Ridge" Thunderbolt controller, which also provides all the USB-C port functionality and is limited to DisplayPort 1.2. I've always assumed that limitation applied to both DP-over-TB (used by Thunderbolt displays & docks) and USB-C DP alt mode (the USB/VESA specs may allow 1.4 over the connectors/cables but the signal still has to get through the Alpine Ridge chip). Is it possible that newer Macs can run DP1.4 over USB-C - or are Moshi jumping the gun on future Macs with the recently released "Titan Ridge" TB3 controller with DP1.4 (effectively "Thunderbolt 3.1" in all but name). Not that the market is bursting with DP1.4 displays that could take advantage of it...

 

[iMacDonald]

I've been using the Plugable USB-C to DisplayPort cable to drive a Dell S2817Q for 6+ months with no problems. Maybe you were unlucky?



Hear, hear!

It's a desktop - plenty of space for a couple of Mini-DP sockets. Shoe-horning multiple functions onto one connector is an unnecessary complication that's only justifiable on tablets and ultra-portables - and unlike Thunderbolt 1/2, the USB-C "alt modes" clearly need more than a simple, physical adaptor to drive DisplayPort devices. Apple couldn't even contrive to have a suitable adapter in their store (...at least not in the UK store, the Moshi cable on the US store looks like a new arrival...)

NB: Irrelevant to your problem, but that Moshi cable is intriguing - 5k support over a single USB-C cable would require DisplayPort 1.4 which I didn't think was supported at all by current Macs, thanks to the "Alpine Ridge" Thunderbolt controller, which also provides all the USB-C port functionality and is limited to DisplayPort 1.2. I've always assumed that limitation applied to both DP-over-TB (used by Thunderbolt displays & docks) and USB-C DP alt mode (the USB/VESA specs may allow 1.4 over the connectors/cables but the signal still has to get through the Alpine Ridge chip). Is it possible that newer Macs can run DP1.4 over USB-C - or are Moshi jumping the gun on future Macs with the recently released "Titan Ridge" TB3 controller with DP1.4 (effectively "Thunderbolt 3.1" in all but name). Not that the market is bursting with DP1.4 displays that could take advantage of it...

I'm back on the Plugable cable at the moment. It did work 10 hours yesterday but then went crazy too. Fingers crossed it keeps working today. It does seem like it's the highest quality cable out of the 3. I was initially impressed with the Rankie till it came to unplugging it. My heart almost sank when I couldn't release it from the iMac for 20 seconds or so.

I'm a bit disillusioned by this and has taken the shine off enjoying my new iMac and to add insult only just noticed the iMac Pro has dropped in price considerably from laptopsdirect to £4200 at the moment.
Perhaps I should return my iMac and get the Pro to have more USB-C ports and a more powerful graphics card to play with going forward.

 

[Fishrrman]

My advice:

DON'T buy a "USB-c" display.
Instead buy a "regular" computer monitor that has connections for displayport and/or HDMI.

THEN...
... get a USB-c to displayport cable and connect to the display that way.
Things will go much better.

A WORD OF WARNING:
NOT ALL USB-c to displayport cables will work properly with the 2017 iMacs (and MBPros). Some will, but others will not (those that will not often work fine with earlier Macs, but not with the 2017 models). You have to "investigate carefully" to find and buy the right one. Read the product descriptions carefully, and check user reviews as well.

A good place to start shopping for displays:
Comprehensive List of S-IPS/H-IPS/e-IPS and other IPS Based LCD Monitors