If you have a Thunderbolt/USB-C dock, how do you switch between two Macs? KVM? Which?

scstr · August 11, 2022, 5:21pm

I have lots of USB-A external drives and devices. Have had a bunch of different hubs over the years. It’s been a trial to troubleshoot USB. But a dock has more reliable power to its ports than a mere hub? Especially for devices that insist on plugging in directly to a Mac to get sufficient power? “You get what you pay for?”

What if I take the plunge and buy a Thunderbolt/USB-C dock from one of the usual suspects, like OWC, Plugable, CalDigit, Sonnet and so on? What if I wanted to switch between a 2018 Mac mini and a 2020 M1 MacBook Air, with all devices plugged into the dock?

How do you switch between Macs? A KVM switch? Which KVM switch do people use in this scenario? Can there be a compatibility issue between a given KVM switch and a given dock?

Any usage examples would be appreciated.

Shamino · August 11, 2022, 8:37pm

I wouldn’t make that assumption.

Yes, some cheap USB hubs don’t include power supplies big enough to supply maximum current to all ports, but that just means you need to do your homework and get one that does.

For example a USB port that supports the basic power capabilities (not the more advanced PD specification) should be able to supply up to 500mA (2.5W) for a USB 2.0 port and up to 900mA (4.5W) for a USB 3.0 port. So if you have a 10-port USB 3 hub, it may need to supply up to 9A (45W) if all the ports are drawing maximum power. If its bundled power brick can’t supply at least that much, you may end up with unreliable power. But if its power supply is big enough, you shouldn’t have any problems.

If your hub supports higher power (via the PD spec), then it should tell you how much it can deliver to each port. Add them all up and make sure its power supply is at least that big.

For example, the USB hub I’m using (Anker AH231) consists of 10 USB3 ports. It is documented to support up to 900mA on 9 ports, with up to 2A on the 10th port, maxing out at 10A across all 10. So in order to deliver the advertised amount of power, its power supply needs to provide at least 50W (10A at 5V). Since it comes with a 60W power supply, it should be able to keep up. And I haven’t had any problems with it.

For docks (whether USB or Thunderbolt), you need to do the same math. Figure out, from the product specs, the maximum amount of power it is able to deliver. Make sure its bundled power supply can provide that much (plus a bit more for overhead and a safety margin) and make sure the devices you connect won’t try to draw more than that. Assuming the device isn’t cheap junk, you should be just fine.

This may be a bigger question than you realize. Are you talking about switching when the computers are powered off or are you talking about hot-swapping them?

You must be careful when hot swapping because some kinds of devices will need special care to prevent system crashes and corrupt data. For example:

A storage device (hard drive or SSD) must be unmounted/ejected from a computer before it or its hub/dock is disconnected. Otherwise you risk corrupted data due to the computer not being able to flush its caches before the disconnect.
If you have an external GPU or other advanced PCIe device connected, it is probably a bad idea to disconnect it without first shutting down the attached computer. Some devices (like eGPUs) may include a utility to let you safely disconnect it, but in the absence of such a utility, I wouldn’t try it.

On the other hand, more mundane devices like keyboard, mouse, network adapter, display, audio, etc. can probably be hot-swapped at will. Just be sure to check the manual for your hub/dock just to make sure. Consider downloading and reading the manual before you buy, so you don’t find any surprises.

As for how to swap devices, there are plenty of USB KVM switches out there. Some simply switch a set of USB ports between two computers, while some include additional connectors to switch video (typically HDMI, but possibly DVI or VGA) and analog audio. As with all things, do your homework - the least expensive model may not work good enough for your needs.

I would be far more cautious about switching Thunderbolt. If the KVM/switch isn’t explicitly designed for TB, you should assume that it won’t work. At least not as a TB switch - if you search for Thunderbolt 3 switches, you find a lot of USB-C switches (I couldn’t find any TB switches when I searched). If you use one of them, your TB3 port will only be operating as a USB port and you’ll lose any TB-specific functionality.

If you need to swap a TB dock between two computers, you might be better off avoiding a switch altogether and just get an extra TB cable and manually connect one or the other computer to the dock’s input port.

tommy · August 12, 2022, 7:33am

Which is what I do, I attached a TB4 male-female short cable to the Caldigit dock so the female end was easily available and I have two short TB4 cables from my two Macs. I shut down and restart in between swaps. I have found that it helps having one set up that does 80-90% of the time, being my MBPro connected to the Dock and my iMac not. I have a good deal of kit on my network, a RAID and printers, the iMac still has access to those. So it is not so bothersome, essentially if I want my iMac to have access to everything, it’s quickly and easily do-able.

gingerbeardman · August 12, 2022, 2:27pm

You might look at a USB-C Bi-Directional Switch that does data, video and power delivery.

There seems to be only one design sold by many brands.

mpainesyd · August 12, 2022, 9:43pm

The (discontinued) Apple Time Capsule would have made a handy device for connecting a range of peripherals to a Mac network. Of course it only had a simple USB port for an external drive and its transfer speeds were woeful but it was a useful for network storage. In fact I still have one on my network but have disabled its wifi function.
Update
On a whim I tried connecting an old USB hub (powered, USB3) to the Time Capsule. Then I connected 2 external (USB-c) drives to the hub. All Macs on the network can see them and access the drives. The transfer speeds are a bit slow (just backing up 300Gb of videos which it reckons will take 5 hours). That is OK for my needs.

RonL_SFO · December 28, 2024, 1:49am

I have a *Cable Matters 14-in-1 Dual 4K 60Hz USB C KVM Switch Dock for 2 Computers, Up to 100W Charging, DisplayPort & HDMI. It connects my 2019 Mac Pro and my work MacBook Pro M2 to my 39" widescreen, keyboard, DAC for audio, and a lot more – except my Mac Pro’s external RAID volumes. I don’t know if it was updated, but the web page says it doesn’t support desktops.

scstr · December 28, 2024, 4:01am

I looked it up.

Fascinating. This is the first time I have seen “KVM Switch Dock for 2 Computers “ as the title of a product. At the time I posed the question, I don’t believe there was such a thing as a “Dock for 2 Computers.” That would have solved my question.

Thanx much for posting.

And clearly Cable Matters is a longtime reputable company with quality products and support.

P.S.
@ace would there be interest in a review of this product for TidBITS? If so @RonL_SFO has real world experience to share.

RonL_SFO · December 28, 2024, 4:34am

I’ve been using it for a year. Connected are my backlit keyboard, Datacolor X2 monitor calibrator, 39” LG widescreen DisplayPort monitor, Polycom Studio P15 videoconference device, Rode USB microphone, and FiiO DAC which drives headphones and an amplifier to my desktop monitors. There are 10 gbps USB-C and USB 3.2 ports, 2 SD card slots and a headphone jack on the front.

Works fine with my desktop.

kat634e · December 28, 2024, 7:20pm

I have had good experiences with IOgear KVMs. I have four older Macs sharing a 4-port DVI KVM, and a 2012 Mini and a Studio (M1) sharing a 2-port HDMI KVM.

Some mice don’t work beyond the basics when attached to a KVM. I use Contour Design mice with buttons customized to my preferences and they have to be connected directly to their respective computers.

Shamino · December 29, 2024, 1:32am

In order to work with two computers without the computers thinking the keyboard and mouse were disconnected, the KVM probably runs its own keyboard/mouse emulation firmware and isn’t just passing the raw USB data downstream.

As such, any data beyond that of the standard keyboard/mouse HID properties may not get propagated.

For a keyboard, this is up to 8 modifier keys (shift, ctrl, alt, GUI for left and right sides), up to 6 simultaneous keypresses, and up to 5 LED indicators.

For a mouse, this is X/Y movement and up to 8 buttons (left, right, center and 5 device-specific). It’s my understanding (based on work with Linux configuration) that mouse-wheels are implemented as buttons 4 and 5.

If your keyboard or mouse have additional features that don’t fit with the standard (e.g. volume knobs, extra mouse axes, more than 8 mouse buttons, support for more than 6 simultaneous key presses, etc.), they will need to be implemented as either device-specific extensions to the protocol or (more likely) as additional USB HID devices, which a KVM probably doesn’t have the ability to track, propagagte or emulate.