Theoretically, How Many USB Devices Can I Connect via a USB Hub?

HappyDude20

macrumors 68030
Original poster
Jul 13, 2008
2,678
191
Los Angeles, Ca
hello,

So I am interested in the latest 2018 rMBP. Considering it has the most usb c ports I was wondering if I was to purchase a usb hub, how many usb devices I could theoretically hook up.

I was daydreaming that it would be cool to arrive home and simply connect my rMBP to one usb c cable that is connected to a hub of say 4-30 usb devices; from speakers, monitors and anything my little technological mind can and may want to connect.

I know there are some limitations based on the computer and devices I have; some accept a 5K display while older Macs support 2 4K displays etc. but doubt it would be a huge power draw if I connected a TwelveSouth BassJump subwoofer and any other unique peripherals I may want to connect to my hub, which would probably rest hidden behind my desk in the family room.
 

dwig

macrumors 6502a
Jan 4, 2015
650
238
Key West FL
In theory...

... USB devices need to be enumerated (assigned a number) to keep them sorted out and the host controller has only 128 "fingers" for the counting. This means that in theory any one host controller can connect to 127 devices, given that one "finger" is needed for itself. Since most computer designs put two USB jacks on each controller that means the controller and jack set may tie up 3 addresses, depending on circuit design, leaving a theoritical 124 for external devices, total for the two jacks. I believe that hubs count as a device as well.

Connecting a large number of high bandwidth devices to a single controller can work but can also end up with bandwidth issues that severely limit functionality.
 

HappyDude20

macrumors 68030
Original poster
Jul 13, 2008
2,678
191
Los Angeles, Ca
In theory...

... USB devices need to be enumerated (assigned a number) to keep them sorted out and the host controller has only 128 "fingers" for the counting. This means that in theory any one host controller can connect to 127 devices, given that one "finger" is needed for itself. Since most computer designs put two USB jacks on each controller that means the controller and jack set may tie up 3 addresses, depending on circuit design, leaving a theoritical 124 for external devices, total for the two jacks. I believe that hubs count as a device as well.

Connecting a large number of high bandwidth devices to a single controller can work but can also end up with bandwidth issues that severely limit functionality.
Thank you for the thoughtful reply.

So, let’s take a 2013 iMac for example that comes with 4 USB ports. Am I correct in understanding that the iMac can connect to 128 devices, minus 3 cause those USB ports on the back of the iMac are tied to addresses? OR, each USB port can accept 127 devices EACH?

If so, would my limitations then be based on what exactly it is I’m trying to connect to my iMac?

How differently is this when it comes to usb c ports?

I guess in theory I’m wanting to connect to a whole lot of devices that will instantly turn on when I arrive home and plug in my 2018 rMBP. Sound systems, high quality monitors and other accessories are the goal ideally.
 

HappyDude20

macrumors 68030
Original poster
Jul 13, 2008
2,678
191
Los Angeles, Ca
I’m wanting to draw up a list of usb devices that I’ll want to have connected to my hub and am both curious as to the total number of devices I’ll be able to connect and also of any unseen problems this can create.

Ideally, I’d want to arrive home and connect my MacBook to a single usb c cable that’s connected to either one hub or many of which have speakers and subwoofer, monitors x3, perhaps an eGPU, and other cool home autism action stuff perhaps.
 

BeatCrazy

macrumors 68000
Jul 20, 2011
1,806
726
I’m wanting to draw up a list of usb devices that I’ll want to have connected to my hub and am both curious as to the total number of devices I’ll be able to connect and also of any unseen problems this can create.

Ideally, I’d want to arrive home and connect my MacBook to a single usb c cable that’s connected to either one hub or many of which have speakers and subwoofer, monitors x3, perhaps an eGPU, and other cool home autism action stuff perhaps.
I think you're overthinking this. The chances of one of those USB devices conflicting with macOS or another connected device and causing problems is a lot higher than the raw "number" of USB devices connected. In other words, you'll have a problem with devices interacting with each other before you run out of bandwidth or controller resources.