New Targus Thunderbolt dock for my MacBook Air M3 (ed)

TidBITS Talk
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After the previous docking setups I was using started exhibiting even more issues (I posted about the first ones around last november), I finally caved in and bought myself a TB dock to see if this would solve all my problems.

Since I can’t afford a costly CalDigit dock and because we use Targus docks at the office (devices that use DisplayLink via USB), I checked to see if they had any TB docks. They do; I bought a DOCK215 that uses ThunderBolt 3. That thing is much less expensive than the CalDigit units others have recommended to me. (FWIW, it does come with a TB3 cable, so no need to buy one separately.)

I removed all traces of DisplayLink manager I had installed previously on my MacBook to make sure it would not get in the way. Despite this, I had a bit of trouble to get the external display working: the Mac didn’t see it at all, despite rebooting/power-cycling the computer and/or the dock, unplugging/replugging the dock from the computer, etc. The display only came to life when I finally unplugged & replugged it whilst the dock was turned on. It’s as if the display controller in the dock needed that to fully initialise or something.

Anyway, despite System Report telling me there is a 40Gb/s connection on bus “Thunderbolt/USB4.0”, I’ve noticed that the dock’s built-in audio I/O and its NIC are listed as USB devices.

Since this is the first ever TB dock I have ever used, I’m not sure if this is normal or if it’s a sign of badly recognised/configured hardware.

So, is it OK or is there a problem with my dock?

Thanks in advance for all your help.

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Not an immediate reason for concern. TB3/4 natively speaks USB3.1 Gen 2 so it makes sense to attach low-b/w peripherals like audio and i/o devices through TB3/4’s USB layer rather than implementing over native PCIe. I’m not aware of a single brand name TB3/4 dock that does not apply the same trick to enable headphone/mic ports or SD card readers etc.

I’d say put the dock through its paces and chances are you’ll be just fine. If it delivers good power to your Mac, outputs video without any weirdness (flicker, going black, refreshes, etc.) and you see speedy USB i/o, you’ll be fine. Enjoy the convenience. I’m not familiar with these Targus units, but I have been very happy with several Elgato, OWC, and CalDigit TB3/4 hubs I have used over the years with various MBPs.

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It’s not been 24 hours since I set up this new TB3 dock and I purposely left my MBA go to sleep multiple time; no issues yet with secondary display (or any other attached peripherals) not coming back up when I wake up my Mac.

Still not sure why a Thunderbolt dock would work better than a USB3+ one, especially when dealing with “waking up” situation, it doesn’t make much sense to me if “the stack” is properly implemented. TB is just the lowest layer and should be no different than plain USB for the layers above. It’s like saying Ethernet works better than, say, ARCnet for TCP/IP when the only difference would be speed (I don’t know if this is the best example, but I hope you get the point).

Knock on wood and let’s hope this continues.

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That assumption isn’t always valid. Due to USB’s extreme popularity, there are a lot of cheap chipsets that may not properly implement the entire stack (especially the less common edge cases or returning the correct error codes for certain situations).

I actually have personal experience with this. Years ago, I installed a USB 2.0 card in my PowerMac G4 (its built-in ports are USB 1.2). I was told that these cards are all pretty generic so it doesn’t matter what model you get. The first one I used was a cheap one with a Realtek chipset. It worked, but the computer would no longer wake from sleep - you needed to power-cycle it after any attempt at sleep. It also would occasionally result in the Finder hanging for devices connected to its ports.

I later replaced that card with a different cheap card with an NEC chipset and everything worked perfectly. And that Realtek card worked fine in a Linux PC (where I assume the open source community found and worked around its bugs).

So I have no problem believing that similar things happen today with the chipsets used in docking stations.

Thunderbolt, on the other hand, is not as common, and the standard is controlled by Intel (who seems pretty litigious), so I doubt there are a lot of knock-off chipsets. I suspect most (if not all) of the big-name device makers are using Intel chipsets, so you’re more likely to get a proper implementation, or at least one where the bugs are well known and have been worked around in system software.

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My “costly CalDigit dock” has the audio I/O and SD card reader as USB devices, although the Ethernet interface is PCI. My (even more costly) Studio Display also exposes the built-in speakers and microphone as USB devices.

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This makes perfect sense to me.

Audio is low-bandwidth, so microphone, speaker and headphone jacks have no need for PCI bandwidth. Even if you’re outputting 5.1 surround at 24-bit, 192 kHz sampling (a pro-audio configuration), the total bandwidth required is still pretty low:

  • 192,000 (samples per second) * 24 (bits per sample) * 6 channels = 27,648,000 bit/s or 26.4 Mibit/s. That’s well below USB 2.0’s high-speed limit of 480 Mbit/s.
  • For something more typical of consumer use, 16-bit, 48 kHz 2-channel (stereo), the bandwidth required is only 48,000 * 16 * 2 = 1,536,000 bit/s or 1.5 Mibit/s. This is well below even USB 1.x’s full-speed limit of 12 Mbit/s. And at CD quality (44.1 kHz), the bandwidth is 1,411,200 bit/s or 1.35 Mibit/s, which might even be achievable over a USB low-speed (1.5 Mbit/s) interface.
  • Just for kicks, assuming a device can get 75% of a USB 2.0 port’s bandwidth (360 Mbit/s), that would be enough bandwidth for over 80 channels of 24-bit 192kHz audio (80 * 24 * 192,000 = 368,640,000 bit/s or a bit less than 352 Mibit/s). Which would be enough for just about any multi-track recording solution you can think of.

An SD card reader is going to be throttled by its media. The fastest SD cards (the V90 spec) top out at 90MB/s (or 720 Mbit/s). This is faster than a USB 2.0 high-speed interface, but is much lower than a USB 3.0 5G SuperSpeed interface.

In theory, a USB high-speed interface of 480 Mbit/s could theoretically move 60 MB/s (a V60 SD card’s full bandwidth) without blocking. In practice, the usable bandwidth wouldn’t be that high, but it should still be able to keep up with the next-lower speed tier (a U3/V30 SD card) of 30 MB/s.

So I’m not surprised that the CalDigit SD card reader is USB-attached either.

See also: A Guide to Speed Classes for SD and microSD Cards - Kingston Technology

For network interfaces, now you need more bandwidth. A Gigabit Ethernet port should work over a USB 3 SuperSpeed interface, but a faster port (2.5G, 5G or 10G) is going to have a hard time gettomg enough bandwidth from USB. A 2.5G interface might work over a 5G USB port and a 5G interface might work over a 10G USB port, but a 10G interface is going to require a 20G USB port. And Ethernet is (usually) full-duplex, so the device needs to send and receive that much data at once (2.5, 5 or 10Gbit/s in and out).

Once you’re looking at those speeds, which are approaching the speeds of Thunderbolt itself, it makes perfect sense to use a PCI connection. Especially when you consider that PCI cards with these interfaces have existed for quite a while, so chips with PCI connectivity are probably more common than those with USB connectivity.

Looking at the specs, the Targus DOCK215 has a Gigabit Ethernet port. So it should be able to comfortably run on a 5G USB interface.

@Incompatible didn’t say which model CalDigit he’s got, but looking at the current product offerings:

  • The TS4 ($380, MSRP of $400) has 2.5G Ethernet.
  • The TS3+ ($210, MSRP of $240) has Gigabit Ethernet.
  • The Mini Dock ($150) has Gigabit Ethernet.

Based on port-speeds alone, I would expect the TS4 to have a PCI-connected Ethernet interface, but the TS3+ and Mini could go either way.

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@Shamino : I see the pricing you listed for CalDigit devices, so I checked how much the DOCK215 sells for in the USA. Surprisingly, it sells for more in the USA (250USD) than it does in Canada (200CAD). Comparatively, CalDigit devices can sell for 3+ times the price I paid for my Targus, new, in Canada. They are well-regarded and are oft-recommended for a reason, but they’re still too costly for me.

Anyway, now I know that my dock is OK and that its devices (audio, NIC) being identified as USB-connected devices (and not thunderbolt-connected) is normal.