Mixing USB device speeds on Pi4

[bob prohaska]

It would be handy to plug a USB2 device into one of the USB3 ports
on a Pi4. The machine has a keyboard, mouse and FTDI TTL232R usb-
serial adapter in addition to the USB3 boot hard disk.

Will using one USB3 port for, say, the high-speed FTDI adapter slow
down traffic to the USB3 hard drive? I gather that at least some
usb devices slow down all ports when one port has a slow device,
but that notion is old and I'm not sure it applies to the Pi4.

The machine is running that way now with no obvious problems, but
hasn't been seriously exercised yet..

Thanks for reading!

bob prohaska

[Theo]

bob prohaska <b...@www.zefox.net> wrote:
> It would be handy to plug a USB2 device into one of the USB3 ports
> on a Pi4. The machine has a keyboard, mouse and FTDI TTL232R usb-
> serial adapter in addition to the USB3 boot hard disk.
>
> Will using one USB3 port for, say, the high-speed FTDI adapter slow
> down traffic to the USB3 hard drive? I gather that at least some
> usb devices slow down all ports when one port has a slow device,
> but that notion is old and I'm not sure it applies to the Pi4.

No - USB 3 SuperSpeed and USB 2 High/Full/Low Speed are essentially
completely separate networks. Separate pins, separate hub components
(inside the same chips, typically).

Some laptops even route the SS and HS pins from one port to different
components - you might have the wifi chip on the SS pins and the camera on
the HS pins, for example.

There shouldn't be any slowdown plugging a HS device into a SS port.

Theo