iPhone 15 Pro Max Thunderbolt 4 hubs work on iPhone 15 Pro Max

[DeanL]

TLDR: I plugged seven devices including a display and three drives to the iPhone. Woah.

Connection Tree

iPhone 15 Pro Max
OWC Thunderbolt 4 Hub

Dell U2720Q USB-C Display​

Yubikey​

USB-C Drive​

Gigabit Ethernet Adapter​

USB-C NVMe Drive​

USB 3.2 SATA Drive​

So today I decided to find the limits of the USB-C port on the iPhone 15 Pro Max and in the process made interesting discoveries.

Why I tried that
I was looking for a USB-C hub that has downstream USB-C ports that carry USB-C DisplayPort Alt Mode.

Let me explain: save for a few niche exceptions, you can’t plug a USB-C display to a USB-C hub because the hub won’t push down the DP signal.

Therefore this connection tree wouldn’t work:

iPhone

USB-C Hub​

USB-C Display​

Only the following would work:

iPhone

USB-C Display​

USB-C Hub​

However, it turns out that with Thunderbolt (4?) hubs (or at least the OWC one), it IS possible to connect a USB-C display to the hub! The hub passes down the DisplayPort signal.

NB: if an engineer can explain my questions below, that would be helpful:

• why USB-C hubs can’t do that
• And why Thunderbolt hubs with a device that isn’t even Thunderbolt can do it

But that’s not it: as the diagram shows, I was able to also connect accessories to the hub of the USB-C display.

There isn’t a tool on iOS to do USB-C speed tests, so what I did is record a AppleProRes video to my NVMe drive while keeping everything else plugged.

During the recording, I did an internet speed test and plugged another drive to see how it would affect the quality of the recording (if the bandwidth decreases, the recording will drop frames)

The only time it dropped frames was when I plugged another drive. The internet speed test was half what I usually get, pointing out to the bandwidth limits of USB 3.2 Gen 2.

Quite spectacular that the iPhone can handle recording ProRes through a Thunderbolt hub that has a display and several drives connected!

 

[ManuCH]

Are you sure that's a Thunderbolt dock? I just tried with my Elgato Thunderbolt 3 dock and the iPhone 15 Pro Max says "Thunderbolt accessories are not supported". The iPhone doesn't have a Thunderbolt interface.

 

[DeanL]

Are you sure that's a Thunderbolt dock? I just tried with my Elgato Thunderbolt 3 dock and the iPhone 15 Pro Max says "Thunderbolt accessories are not supported". The iPhone doesn't have a Thunderbolt interface.

Mine is a Thunderbolt 4 dock, so maybe it's Thunderbolt 3 docks that do not work. I've updated to reflect that.

 

[ManuCH]

Mine is a Thunderbolt 4 dock, so maybe it's Thunderbolt 3 docks that do not work. I've updated to reflect that.

That's very interesting and it could indeed be the reason! I think I will order that OWC dock, it seems to be a good one. Are you generally satisfied with it?

 

[DeanL]

That's very interesting and it could indeed be the reason! I think I will order that OWC dock, it seems to be a good one. Are you generally satisfied with it?

I've only had it for a few hours so I need more time to test!

 

[- rob -]

I asked chatgpt 4 to compare TB 4 and 3 and then pasted in OP’s post and asked if to contextualize it.

I am not an expert on this but it does answer the questions OP posed, the who convo is worth reading:

ChatGPT

A conversational AI system that listens, learns, and challenges

chat.openai.com

Here’s a sample:

Downstream Ports and Display Support:
Thunderbolt 4’s capability to support USB-C DisplayPort Alt Mode downstream from the hub is critical here. Unlike traditional USB-C hubs, which typically don't pass through the DisplayPort signal, Thunderbolt 4 hubs are designed to maintain these signals over various connections.

Bandwidth: The fact that recording to the NVMe drive was largely unaffected until an additional drive was plugged in showcases the high bandwidth capabilities of the Thunderbolt/USB-C interface. Thunderbolt 4 maintains a 40Gbps data transfer speed, which would be sufficient for recording AppleProRes while handling other devices. However, when additional storage is accessed, it likely dips into the same bandwidth pool, leading to potential frame drops if the bandwidth is exceeded.

Accessory Support: Being able to connect and use multiple devices simultaneously without much degradation in performance until bandwidth limits were reached also demonstrates the efficiency of Thunderbolt 4 in managing multiple connections and maintaining high throughput.

Bandwidth Limitation Indication: The user’s internet speed test provides an indirect indication of bandwidth utilization. A decrease in available bandwidth during simultaneous device operation suggests the interface is operating at or near its capacity.

The questions raised by the user at the end:
- USB-C hubs usually don't support DisplayPort Alt Mode for downstream ports due to the complexity and cost of the electronics needed to manage different signal types and directionality. It's often a matter of how the hub is designed and what protocols it supports …

 

[ManuCH]

Somehow I think OWC is doing something funky with that hub, different from other Thunderbolt docks. I think it's not making all the signals pass through a Thunderbolt chip, and is acting more as a USB-C hub, with Thunderbolt pass-through capabilities. That would also explain the cheap price compared to all other Thunderbolt docks.

Not that it's a bad thing!

But that's just speculation, as I didn't really do any deep research on this. But for sure it's very interesting.

 

[JPack]

1. Not all USB-C hubs can do that likely because your monitor requires DP 1.4. Not all hubs can do 1.4.

2. Current TB docks can work with non-TB hosts. There are three generations of TB chipsets - Alpine Ridge, Titan Ridge, and Goshen Ridge. Alpine Ridge requires a TB host.

 

[JPack]

Are you sure that's a Thunderbolt dock? I just tried with my Elgato Thunderbolt 3 dock and the iPhone 15 Pro Max says "Thunderbolt accessories are not supported". The iPhone doesn't have a Thunderbolt interface.

Your dock uses Alpine Ridge which means it cannot downgrade to talk with a USB host. Newer chipsets can.

 

[DeanL]

I asked chatgpt 4 to compare TB 4 and 3 and then pasted in OP’s post and asked if to contextualize it.

I am not an expert on this but it does answer the questions OP posed, the who convo is worth reading:

ChatGPT

A conversational AI system that listens, learns, and challenges

chat.openai.com

Here’s a sample:

Thanks-but it's not very useful because the iPhone doesn't officially support Thunderbolt 4, and the response assumes that the iPhone supports Thunderbolt.

I'll wait for an actual engineer to break it down

 

[DeanL]

1. Not all USB-C hubs can do that likely because your monitor requires DP 1.4. Not all hubs can do 1.4.

2. Current TB docks can work with non-TB hosts. There are three generations of TB chipsets - Alpine Ridge, Titan Ridge, and Goshen Ridge. Alpine Ridge requires a TB host.

1. It doesn't have to do with DP 1.4-most (and when I say most, I mean like 99.9% of docks), it just has to do with DP apparently. They do not pass down any of the DP signals for some reason I don't know but would like to know.

2. Right–so if it can downgrade and speak to non-TB hosts, what other Thunderbolt functionalities does it keep in that downgraded mode? Because that means there might be other capabilities we haven't found yet!

 

[tvv]

https://apps.apple.com/us/app/owc-drive-speed/id6470703928 here is an app to run drive speed tests directly from your iPhone.

The OWC hub is TB4 and USB4. USB4 is backwards compatible with USB3. Hence why the hub works just fine.

 

[ctjack]

Hey OP, big props for reviving my old curiosity to TB docks.

I just googled and it looks like TB4 should come with dp alt mode by standard. Or at least anything using the same intel as in OWC4(caldigit element for example) should be able to do the same.

 

[joevt]

NB: if an engineer can explain my questions below, that would be helpful:
why USB-C hubs can’t do that
And why Thunderbolt hubs with a device that isn’t even Thunderbolt can do it

A USB-C hub that supports USB 3.x will get 2 lanes of DisplayPort and a USB 3.x signal from the upstream host (iPhone in your case). The DisplayPort lines usually go directly to a DisplayPort or HDMI port. The USB-C hub would need some extra electronics to automatically switch the DisplayPort from the DisplayPort/HDMI port to a downstream USB-C port or the USB-C hub would need to include a Display Port MST hub to allow connections to multiple ports including a downstream USB-C port.

A Thunderbolt 4 dock connects to the iPhone as a USB-C hub, but it is smarter about how it connects the DisplayPort signals.

The CalDigit Element Hub is a Thunderbolt 4 hub that can work as a USB-C hub and can allow connecting a USB-C display (it has no DisplayPort or HDMI port). A Thunderbolt 4 hub is basically a Thunderbolt 4 dock with fewer ports. I can use a CalDigit Element Hub to fix the crappy USB-C output of a Club-3d CAC-1336 HDMI to USB-C adapter (which outputs USB 2.0 + 4 lanes of DisplayPort).

A Thunderbolt 3 dock should work as a USB-C hub if it has a Titan Ridge Thunderbolt controller. The HP Thunderbolt Dock G2 has a Titan Ridge Thunderbolt controller and it also has a DisplayPort MST hub so it can connect multiple displays using USB-C, DisplayPort, or VGA but macOS doesn't support MST for multiple displays - it will just mirror all the displays.

 

[DeanL]

Dan Charlton (the reference for USB-C/Thunderbolt hubs) has provided the very useful response below regarding why most USB-C hubs don’t pass DP to the downstream ports. TLDR: it has to do with cost.

There is no technical restriction regarding USB-C hubs supporting DP alt mode on the downstream ports – it’s just that there is a small market for such a device since the implementation is ~2X as expensive as a data-only design. There are a few hubs on the market that do this but are not broadly available (yet). As you’ve found, newer Thunderbolt 3 hubs based on Titan Ridge (JHL7440), and all Thunderbolt 4 docks based on Goshen Ridge do pass along DP alt mode over at least one downstream USB-C port for backwards compatibility. Earlier Thunderbolt 3 docks (Alpine Ridge) do not have this compatibility mode.