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If I understand correctly, if you connect an external monitor to a Mac there is more than one way the signals can be sent to the monitor.

It could be HDMI, it could be USB Two-lane Displayport, it could be USB four-lane DisplayPort Alternate Mode, it could be Thunderbolt Displayport, it could be Displaylink.

Is there some place on the Mac it will tell me which mode is being used?

WGroleau
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gman
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1 Answers1

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Yes. System Report. Click on the Apple Logo (top left of Menu Bar) *About this Mac → System Report → Graphics/Displays

System Report

That said.....

It could be HDMI, it could be USB 2 lane Displayport, it could be USB 4 lane DisplayPort Alternate Mode, it could be Thunderbolt Displayport, it could be Displaylink.

You're confusing/conflating a bunch of things all into one big lump. Let's see if we can sort through the confusion - and yes, the industry doesn't help at all in making things less confusing.

The technology...

  • USB-C describes the physical port attributes (connector type) not the actual port itself. It's actually a USB 3.x port or a Thunderbolt 3 port with a Type C connector, but we've been saying USB-C for short; and yes, doing things this way made everything overly complex and confusing

  • Your Mac (assuming 2018 or later because you referenced USB-C), natively outputs a DisplayPort signal. Period.

  • "Lanes" just refers to the bandwidth of the USB or Thunderbolt signal.

  • USB Alt or Alternate Mode is when the DisplayPort signal is "added" or embedded in the USB signaling. USB by itself doesn't have display capability so if your computer manufacturer enabled this capability (i.e. Apple 12" MacBook), you will be able to connect an external display to your MacBook via the USB-C port. From the Texas Instruments PDF linked above:

    This configuration enables other protocols such as DisplayPort (DP) ... ... to transfer over Type-C cables

  • Thunderbolt is actually several signals all in one: PCIe, USB 3 (soon will be 4.0), DisplayPort 3.2 and Power Delivery. If you connect your external display to a Thunderbolt 3 port, you're still connecting it to DisplayPort.

  • DisplayLink is a technology that allows manufacturers to embed the video signals on USB/Thunderbolt, Ethernet and WiFi connections. This is something at the manufacturer level and really, the only time consumers actually get involved with this is when Apple doesn't include the drivers or there's an issue with it supporting something like daisy-chaining.

What you connect to...

If I understand correctly, if you connect an external monitor to a Mac there is more than one way the signals can be sent to the monitor.

Not exactly. Generally speaking there's only one way signals are sent to your Mac and that's from a DisplayPort signal. There are Macs with HDMI ports (Mac mini for, example) but since we're referring to the USB-C port - there's only one connection - DisplayPort.

Your end device (monitor) may be HDMI, however, and depending on how that signal was converted, it my be unidirectional. In other words, it won't communicate back that it's using HDMI.

Yes, you can use a USB-C to HDMI or DVI or even VGA adapter. The DisplayPort signal can be converted to these formats with passive or active adapters (I suggest active).

From the Comments....

It would seem it would be nice to know is it (a) 2 lane and so I'm limited to 30hz at 4k (b) 4 lane and so all my USB bandwidth is being eaten and external storage will be slow, or (c) thunderbolt and so everything is good.

That's not how it works. It's not just one big pipe where everything is shoved through and if you use so much of USB you reduce your video or frame rate.

The "lanes" that you're referring to have to do with the data lanes that can be sent. This is not something that can modified nor utilized by the user in any way. If you have a USB 3.1 Gen 1 port, using a USB 3 cable with "4 lanes" isn't going to improve your video or USB throughput in any way. What you need to focus on is the specifications of your hardware. Looking at the specs

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If your Mac has stated specifications, it has to meet those specifications. Your peripherals also have to meet these specifications. Whether you have 2 lanes or 4 lanes makes no difference.

Allan
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  • . Given there are 3 types of Displayport is that telling me it could be any of those 3 or specifically one of those 3 and it would display something different for the other 2 types of Displayport? – gman Aug 06 '20 at 16:52
  • Where do you see "three types of displayport"? – Allan Aug 06 '20 at 16:53
  • I listed them in my question. As an example most USB-C hubs only support 2 lane Displayport so if you have a 4k monitor you'll get at most 30fps. – gman Aug 06 '20 at 16:55
  • See my answer where I said "you're confusing/conflating...." There's only one DisplayPort. There are versions where it evolved to add more capabilities/speed/bandwidth, but there's only one DisplayPort. – Allan Aug 06 '20 at 16:58
  • yes, and the entire point of my question is to find which version is being used. It sounds like the answer is "no" it won't tell me. It would seem it would be nice to know is it (a) 2 lane and so I'm limited to 30hz at 4k (b) 4 lane and so all my USB bandwidth is being eaten and external storage will be slow, or (c) thunderbolt and so everything is good. Plugging in via a hub I know does not support 4k at 60hz I see nothing in System Report that tells me I'm using a slow connection. – gman Aug 06 '20 at 17:02
  • This is becoming a perfect example of question scope creep and somewhat of an XY Problem. There's a fundamental misunderstanding of the technologies on your part because 1) these aren't options you get to choose, b) if things worked the way you described, it would all be out of spec, and c) "Thunderbolt = everything is good" makes no sense whatsoever (see my bullet point on Thunderbolt) – Allan Aug 06 '20 at 17:11
  • The XY problem is you're interpretation. My question is simple. To rephrase this question. Imagine you have 3 USB hubs unlabeled. Only one of those hubs supports 4k at 60fps. You only have a 2k monitor locally so you can't test by using a 4k monitor. So you plug each each hub one at a time between your Mac and your monitor. How do you tell which one is the 4k/60fps capable hub? Note that many monitors have hubs built in so this is really the same as plugging directly into the monitor. There is nothing XY about wanting to know which type of connection is being used which is what I asked. – gman Aug 06 '20 at 17:14
  • Does your network switch tell you that it's gigabit capable when you only have a 10/100 card in your computer? Does your computer know it's using a CAT5 or CAT5e or CAT6 cable? The way you've phrased your question emphasizes my point that there's a fundamental misunderstanding of the technology. No USB hub is capable of 4K at any FPS. That's like saying which car is capable of 30 knots at a 2m depth. I've repeatedly said Thunderbolt is 4 different signals including DisplayPort and the fact you're ignoring that is baffling. – Allan Aug 06 '20 at 17:24
  • I'm baffled you've ignored the question and are getting angry about it. If I asked "can my computer tell me if it's using CAT5, CAT5e or CAT6 and it can't then the answer is "no it can't" not "here's some other info that doesn't answer the question". As for USB-C hubs that support 4K there are plenty of them. I own one. They are not thunderbolt. I own one of those as well. The thunderbolt hub cost > $300. The USB-C hub only about $120. It does do 4k at 60fps vs the 2 I bought before that that don't. – gman Aug 06 '20 at 17:58
  • No, you put a whole bunch of things in there that made no sense. I'm trying to fix your confusion so you don't keep going around trying to solve a problem that actually doesn't exist. For instance, what you own is a USB-C to HDMI (or something else) adapter with USB-A/SD card, or whatever. There are no Thunderbolt hubs - there are thunderbolt docks. The Thunderbolt dock has more capability because it includes the PCIe bus. But you seem think you need to know how many lanes you have/use for some reason. – Allan Aug 06 '20 at 18:05
  • Here's an article on it. Feel free to point out why it's wrong. It contradicts what you're saying and makes it very clear that there are 5 types of ways for video to get from a computer to a monitor over a USB-C/Thunderbolt cable. It even gives lists of devices that use each method. My question was and still is, can my Mac tell me which 1 of those 5 ways is being used. – gman Aug 06 '20 at 18:12
  • The article isn't wrong. You're misunderstanding it. They're explaing the underlying technologies that make it possible to send video over a USB/Thunderbolt port. Read the part of my answer re: specifications and capabilities. Whether you have 2 lanes or 4 is irrelevant to you. It's also irrelevant to whether or not you still have enough bandwidth to transfer files. – Allan Aug 06 '20 at 18:26