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You are here: MacNN Forums > Hardware - Troubleshooting and Discussion > Mac Desktops > Mini usually can't find second display

Mini usually can't find second display
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sra
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Mar 13, 2022, 12:02 PM
 
My 2018 mini increasingly doesn't find my second display on rebooting, restarting, or waking from sleep. I have a 34" LG ultrawide as primary display, and a 24" LG as secondary. Primary display connected via thunderbolt3, secondary either as HDMI or thunderbolt3-displayport (doesn't seem to make a difference which: when using thunderbolt3-displayport, the two displays are on different busses, ports 1 and 3 on back). Sometimes both show up, but more commonly only the primary, with the secondary one lighting briefly and then going dark, into power-saving mode. Occasionally after a few minutes both go dark and then both wake up. This is driving me nuts: is the problem likely to be (a) the second display itself? (b) the mini? (c) the OS [currently Monterey 12.2.1, but the problem started before updating to Monterey]? What other information can I provide to help nail this down?
     
Thorzdad
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Mar 13, 2022, 01:28 PM
 
If you disconnect both displays, and then reconnect the secondary display only, does it still not work?
     
sra  (op)
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Mar 13, 2022, 01:38 PM
 
As I say, the problem is intermittent. Sometimes both displays work, sometimes I get only the primary one. The issue seems to be the same regardless of whether the second display is connected via HDMI or Thunderbolt3-displayport: sometimes both, but often only the primary. If I have only the second display connected, it works (as far as I can tell -- not a huge number of tests...).
Question: the Apple docs say "You can connect one 5K display using a Thunderbolt 3 (USB-C) port, or up to three 4K displays using two Thunderbolt 3 (USB-C) ports and the HDMI 2.0 port." Does this mean that you can only use a second Thunderbolt 3 port for a third display, assuming one already on a first Thunderbolt 3 port and one on HDMI? Or does it just mean that if you want to connect three displays you have to have one of them on HDMI? I found that to the extent I can get the second display to work, it works more or less equally as well regardless of whether it's connected as a second Thunderbolt 3(-displayport) device or on HDMI, but a commenter on another board suggested that a second display MUST be connected via HDMI.
     
reader50
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Mar 13, 2022, 02:22 PM
 
Originally Posted by sra View Post
Does this mean that you can only use a second Thunderbolt 3 port for a third display, assuming one already on a first Thunderbolt 3 port and one on HDMI? Or does it just mean that if you want to connect three displays you have to have one of them on HDMI?
What Apple's note means:
• If you have a 5K display, you need to use a TB3 port to actually get the 5K resolution. Using the other ports, you'll only get 4K - or possibly 5K at a low refresh rate.
• 4K displays (or less) can use any combination of the ports.

It sounds like your secondary display isn't being identified fast enough. The next time it happens, try this:

Open System Preferences -> Displays
Hold down the Option key - a button will appear on lower right that says [Detect Displays]. Click that - it makes the OS recheck for connected displays.

If it works every time (2nd display comes back) then the above theory is probably correct. 2nd display isn't being identified fast enough for the OS.
If it doesn't work (2nd display doesn't come up) that points to a hardware issue.

You could also swap monitors if you have a spare. Substitute the spare for your current 2nd monitor. See if the problem goes away.
     
sra  (op)
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Mar 13, 2022, 03:43 PM
 
Thanks, reader50. That clears things up. I had noticed that when both displays were connected from TB3 ports, the visual quality of the larger (5K) one seemed a bit degraded by comparison to the TB3+HDMI combination.
I'll see if the "Detect Displays" option, which I had missed, helps the next time I lose one. Sounds right.
     
ghporter
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Mar 15, 2022, 11:27 AM
 
And just for clarification, if you connect JUST the “secondary” display - to any port at all - is it ALWAYS detected? Is it detected every time? Is it detected regardless of what port it’s connected to?

What are you using this second display for? Is it extending your desktop, or showing something else?

Both of these make a difference. The first because there could be some driver issue and/or electronic issue with that second display. The second because how the OS uses a display can affect when (and maybe how?) it tries to connect to it.

Glenn -----OTR/L, MOT, Tx
     
   
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