MP 1,1-5,1 Weird issue! KB/Mouse doesn't work w/ built-in ports, but works with 20" ACD

[rm5]

Yet another bewildering issue with my 5,1. So, I can plug in a keyboard/mouse to the USB ports on the cinema display (20 inch), and they work fine. I just plug 'em in, and connect the USB "section" of the cinema display cable to one of the ports on the 5,1. BUT, when I connect the same keyboard/mouse to the built-in ports, it doesn't work at all—nothing from either the KB or the mouse. Any ideas why?

 

[tsialex]

You did not wrote the macOS release that you are running, but Big Sur and newer macOS releases have extremely poor support for MacPro5,1 ancient USB controllers (Intel OHCI USB controllers) and hot-plug is not supported anymore for any USB 1.0/1.1/1.1a devices - keyboard/mouse/HID devices usually are USB 1.0/1.1/1.1a.

If you connect your USB 1.0/1.1/1.1a device via a USB 2.0 or 3.0 hub, to force a EHCI connection instead of a OHCI one, then hot-plug works again. That's how/why the display USB ports, a USB hub, still work for hot-plugging and it's the recommended workaround for MacPro5,1 USB problems with macOS releases post Catalina.