iMac 5k random Thunderbolt Display port flashing ext monitor

[rorton]

Ha have a 2015 5k iMac from new, just bought 2 4K ext monitors to run from it.

I seem to have a problem with one of the thunderbolt ports, when a monitor is connected to this one port, it drops the display frequently, the other port works perfectly fine

I’ve swapped monitors, and connected the one to the faulty port to the working port, and it’s perfectly fine, plug the other monitor that was in the good port into the other port, and same problem

I’ve unplugged one monitor completely, and just plugged one into the faulty port in case it occurred when running 2 monitors, and was fine when running one, but it still occurs. Tried this with both monitors and same occurs

Anyone seen this happen, sound like a faulty port?

 

[mikehalloran]

Although you have 2 TB ports, you have only one bus. Video goes through that bus. Unless using TB monitors, the ports are Mini Display Port for video.

You don’t mention the monitors you are using. Some of them require HDMI resolutions and cables that your iMac does not support.

You also haven’t mentioned how you are connecting these monitors. DisplayPort is more forgiving than HDMI. MDP to DP cables ($8–$20 on Amazon) may fix you up. If you’re connecting HDMI, that’s what I would try first.

If that doesn’t do it, change your resolution to 1080. See if that makes the problem go away. If so, your GPU - which you don’t mention – isn’t up to the resolution you have selected.

 

[rorton]

Although you have 2 TB ports, you have only one bus. Video goes through that bus. Unless using TB monitors, the ports are Mini Display Port for video.

You don’t mention the monitors you are using. Some of them require HDMI resolutions and cables that your iMac does not support.

You also haven’t mentioned how you are connecting these monitors. DisplayPort is more forgiving than HDMI. MDP to DP cables ($8–$20 on Amazon) may fix you up. If you’re connecting HDMI, that’s what I would try first.

If that doesn’t do it, change your resolution to 1080. See if that makes the problem go away. If so, your GPU - which you don’t mention – isn’t up to the resolution you have selected.

thanks for replying

The monitors are AOC U2777PQU 27inch 4K - they have HDMI and Display port

Im driving them via display port, so mini display port to display port cable in each thunderbolt socket on the iMac into the monitors.

GPU is AMD Radeon R9 M395 2 GB

What odd is that with only one monitor plugged into the 'suspect' port, and nothing in the other, it flickers, plug this into the other port - and its stable.

 

[mikehalloran]

Before condemning this as a hardware problem with your iMac, look at the simple stuff.

If using adapters, get cables. Many have reported problems with adapters — worst case, you return them to Amazon.

When plugging multiple devices into TB ports, it does make a difference which port is which. Apple has support docs on this. If it’s one monitor only, it’s not supposed to make a difference. So, a hardware issue is possible, not very likely.

Then there’s your resolution and refresh rates which do make a difference. If you’re trying to get better than 60Hz refresh on a 2015, that’s not going to work very well. Here’s the support doc from Apple:
https://support.apple.com/en-us/HT206587