This weekend I found myself pleased to receive a Kensington SD5000T Thunderbolt 3 Docking Station.
Some of its functionality was a bit of a weird surprise.
Due to my ... accretive history with computer purchases, I have 3 things on my desk at home: a USB-C macbook pro, a 27" Thunderbolt iMac, and an older 27" Dell display, which is old enough at this point that I can’t link it to you. Please do not take this to be some kind of totally sweet setup. It would just be somewhat pointlessly expensive to replace this jumble with something nicer. I purchased the dock because I want to have one cable to connect me to power & both displays.
For those not familiar, iMacs of a certain vintage1 can be jury-rigged to behave as Thunderbolt displays with limited functionality (no access from the guest system to the iMac’s ethernet port, for example), using Target Display Mode, which extends their useful lifespan somewhat. (This machine is still, relatively speaking, a powerhouse, so it’s not quite dead yet; but it’s nice to be able to swap in my laptop and use the big screen.)
On the back of the Thunderbolt dock, there are 2 Thunderbolt 3 ports. I plugged the first one into a Thunderbolt 3 to Thunderbolt 2 adapter which connects to the back of the iMac, and the second one into the Macbook directly. The Dell display plugs into the DisplayPort; I connected my network to the Ethernet port of the dock. My mouse, keyboard, and iPhone were plugged into the USB ports on the dock.
I set it up and at first it seemed to be delivering on the “one cable” promise
of thunderbolt 3. But then I switched WiFi off to test the speed of the wired
network and was surprised to see that it didn’t see the dock’s ethernet port
at all. Flipping wifi back on, I looked over at my router’s control panel
and noticed that a new device (with the expected manufacturer) was on my
nmap seemed to indicate that it was... running exactly the network
services I expected to see on my iMac. VNCing into the iMac to see what was
going on, I popped open the Network system preference pane, and right there
alongside all the other devices, was the thunderbolt dock’s ethernet device.
The Punch Line
Despite the miasma of confusion surrounding USB-C and Thunderbolt 32, the surprise here is that apparently Thunderbolt is Thunderbolt, and (for this device at least) Thunderbolt devices connected across the same bus can happily drive whatever they’re plugged in to. The Thunderbolt 2 to 3 adapter isn’t just a fancy way of plugging in hard drives and displays with the older connector; as far as I can tell all the functionality of the Thunderbolt interface remains intact as both “host” and “guest”. It’s like having an ethernet switch for your PCI bus.
What this meant is that when I unplugged everything and then carefully plugged in the iMac before the Macbook, it happily lit up the Dell display, and connected to all the USB devices plugged into the USB hub. When I plugged the laptop in, it happily started charging, but since it didn’t “own” the other devices, nothing else connected to it.
This dock works a little bit too well; when I “dock” now I have to carefully plug in the laptop first, give it a moment to grab all the devices so that it “owns” them, then plug in the iMac, then use this handy app to tell the iMac to enter Target Display mode.
On the other hand, this does also mean that I can quickly toggle between “everything is plugged in to the iMac” and “everything is plugged in to the MacBook” just by disconnecting and reconnecting a single cable, which is pretty neat.