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The setup:

  • a late-2013 15-inch MacBook Pro; no dGPU
  • at work: two identical 1080p monitors. One connected via HDMI through a Belkin Thunderbolt 2 dock, another directly via HDMI
  • at home: two identical 1680x1050 monitors. One connected via HDMI to DVI cable, another via mini-DisplayPort to DVI cable.
  • 10.5.5 Beta (19F83c)

My work setup works fine.

But with my home setup, no matter which external monitor I connect first, as soon as I connect the second, I don't get a stable image. Instead, I mostly get three black screens, but every ~5 seconds, I briefly get either some or all of the displays briefly displaying an image; then it disappears again. Connect just one of the two, and it works fine.

It can't be bandwidth, because surely, 1680x1050 monitors take up less bandwidth than 1080p ones. I've also tried reducing one monitor's resolution before connecting the other, to no avail. It really feels more like one of them pesky Catalina issues.

I've tried:

  • resetting the PRAM
  • resetting the PMU
  • deleting the ByHost preferences (which did contain many of the display setups)
  • closing the lid so only the two externals are connected

Nothing can get both of those to show a stable image.

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You need an active HDMI adapter.

The signal that comes out of your Mac is DisplayPort. It has to convert the signal to HDMI to make it work.

  • At work, your dock is creating that HDMI signal so in effect, it's got an active adapter built in.
  • At home, you've got an "HDMI to DVI" cable meaning the pins just get re-orderd, but the signal doesn't get properly recreated.

To get the best reliability, try to avoid HDMI altogether. It's a "consumer electronics" standard and (IMO) not up to the bar with respect for computing.

As for connecting/converting signals, the order or reliability is as follows:

  • DisplayPort → DisplayPort. It's the native signal coming out of your Mac, so no conversions mean better reliability
  • DisplayPort → DVI. DVI signaling is very compatible with DisplayPort and many times an active adapter isn't necessary, though I personally still use one because I don't like dealing with these issues once I have all my cabling in place.
  • DisplayPort → DVI. The signals are different, especially the clock signal, which is where a number of issues arise. You can "fix" it with a reboot which allows the signals to "sync" up. But disconnect/reconnect and a reboot is again necessary.
  • HDMI → DVI. IMO, this should be avoided, but if you must convert these signals, an active adapter is really the only way to ensure reliable connections.

Note: These issues aren't as prevelant in Windows because the drivers for the adapters seem to have more tolerance for the signaling discrepancies. macOS, has much less tolerance, thus the need for quality active adapters.

If you can swing it, I would get a dock similar to the one you have at work. It will get you the best reliability and most flexibility.

Additional Reading

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  • Then why does it work if I only connect the screen that goes mini-DisplayPort-to-HDMI? It only fails if I connect both at a time.
    – Sören Kuklau
    Commented May 11, 2020 at 18:01
  • Clock signaling. See my answer.
    – Allan
    Commented May 11, 2020 at 18:10

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