Apple Silicon Macs will not drive 2 external monitors

[burdett]

I recently discovered that all the Apple Silicon Macs will not drive 2 external monitors. 😞 

According to CalDigit (my preferred Mac dock) This is not a limitation of CalDigit’s Thunderbolt or USB-C docks, but of the Thunderbolt 3 ports on the new M1-based Macs.  

A workaround is to have the 2nd external monitor driven using 3rd-party DisplayLink drivers: https://www.displaylink.com/downloads/macos

You'll need a Displaylink-certified USB-to-HDMI adapter. I believe this Startech adapter will work, USB32HD4K USB to HDMI 4K adapter: https://www.startech.com/en-us/audio-video-products/usb32hd4k,   Also the Dell thunderbolt dock may have a Displaylink-certified output so may just need the DisplayLink drivers.  I need to test to verify both of these.

[geoff_widdowson]

Just the other day I was testing this myself. I had a DisplayLink USB-A to HDMI adapter, that I have had for years. I was able to use this plugged into a USB-C mini hub into one of the USB-C ports on my 13" MacBook Pro M1. In the other USB-C port I used an official apple USB-C to HDMI adaper. Both my HDMI monitors worked. The real trick is buying an adapter that requires DisplayLink, as they don't always state that when you buy them. 

[mschroder]

I seem to remember that also Intel Macs can't drive two monitors via one USB-C cable, as it is really a restriction of the thunderbold port. But you can use a second USB-C (or thunderbold) port for a second screen.

[jrippy]

@burdett Unfortunately this is a known limitation with all M1 Macs, with the exception of the Mac Mini that can also drive a monitor via it's built-in HDMI port.

@mschroder For Intel machines with TB3, they can drive 2 and sometimes 3 monitors natively.  This includes 2 via a single TB cable using a TB dock or using a USB-C cable with a USB-C dock meant for Apple computers (verified with an OWC USB-C Dock).  If the dock is meant for Windows machines, the manufacturer will tend to implement DisplayPort Multi-Stream Transport (MST).  MacOS does not support this mode of DisplayPort so these docks do not work with any Apple machine other than mirroring the displays.

I hope this shortcoming is addressed in the M1X/M2 chips.

[AJPinto]

It has baffled me this entire time that Apple refuses to support DisplayPort MST. Apple claims to fully support the DisplayPort standard but lacking MST alone proves this is a lie. I use a displaylink dock myself so the multi display thing is not an issue for me but it is really annoying that I need to install software and give it permissions every time I reimage my M1 test mac. On Windows you dont actually need the software either, I bet it uses MST by default lol.

The intel Macs will support 1 display per thunderbolt port. Some shenanigans can be done to get more displays than that as Intel sets the limit not Apple. 

I wonder if the M1x (or whatever they are called) will get the TB4 standard. That would require support for 2 4k display's per thunderbolt port, as well as a host of other intel specific features. 

[jrippy]

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@AJPinto wrote:

The intel Macs will support 1 display per thunderbolt port. Some shenanigans can be done to get more displays than that as Intel sets the limit not Apple. 

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For this, I don't know what to say, except I've always been able to drive 2 4K displays with my 2019 13" MBP over a single TB cable to a TB dock.  It is specifically NOT DisplayLink and NOT MST.  I assume it is sending everything as Thunderbolt with 2 discrete DP streams encapsulated in it (as TB does), then via whatever chip is in the dock, it splits those streams out from the TB stream and pushes them to the displays properly.  They do show up as discrete displays.

[AJPinto]

Most monitors and docks use DisplayPort to handle their video feed, even if they are thunderbolt monitors or docks. DisplayPort uses MST for multiple Displays on one port (daisy chaining or dock). Some Monitors and Docks truly use thunderbolt for their display signal. Things like Apples Pro Display, LG's ultrafine displays, Belkin docks to name a few use thunderbolt to carry the display signal and not DisplayPort. Its more to do with the technology being used to deliver the display signal then the cable being used. If I am not mistaken even Apples HDMI adapter actually uses DisplayPort, DisplayPort is really the de facto display standard not thunderbolt. Apple just missed the memo while on the ThunderBolt band wagon like 10 years ago.

Thunderbolt 3 requires 1x 4k display per port, but that is the minimum not maximum. Typically if you are using the Thunderbolt Display, each thunderbolt controller supports so many displays. In the case of the MBP13 2x thunderbolt model has 1 thunderbolt controller. The MBP16 has 2x thunderbolt controllers. The number of thunderbolt controllers really comes in to play with the 5k+ displays as apple is sharing both thunderbolt ports display bandwidth on a single controller to drive the display. If the resolution is low enough I could see a single port supporting 4 or more displays. 2x 4k displays seems totally within spec to me if the display technology being used is thunderbolt. Everything aside of macOS could do the same thing through DisplayPort.

[jrippy]

@AJPinto Thanks for that.  Didn't know the info about single vs dual controllers!

[ZenAtWork]

Um.... my M1 Mac Mini is driving 3 right now, albeit with the use of one of OWC's docking station (so therefore two total cables running to the mac itself). It's worth noting that, between the three displays, only one is TB3, too.