I am trying to provide an external 5V power supply to a (previously) bus-powered USB hub.
My first attempt at this was to just connect the USB host's data pins to the hub's data pins, and the external 5V to the hub's power pins, like so:
PSU 5V ----- Hub VCC In PSU GND ----- Hub GND In Host D+ ----- Hub D+ Host D- ----- Hub D-
However, the hub (and attached devices) were not detected by the host PC. My assumption was that as the Host GND and Hub GND were no longer connected, the levels were off, so I added that connection:
PSU 5V ----- Hub VCC In PSU GND ---=- Hub GND In Host GND--/ Host D+ ----- Hub D+ Host D- ----- Hub D-
Still no luck. I then read that a USB host expects a device to draw around 100mA in order for it to be detected, so assuming that the host would output 5V, I added 50R of resistance between it and the host GND so that 100mA would always be drawn:
PSU 5V ------ Hub VCC In PSU GND ----=- Hub GND In Host GND -=/ | 50R | Host 5V -/ Host D+ ----- Hub D+ Host D- ----- Hub D-
In this configuration, with the host PC and the PSU turned on, I measure 70 mV across the resistor and zero resistance. When I unplug the USB cable from the host, I measure the 50R properly.
Am I missing something here? Why would the USB host not detect the hub, and why would it appear to short its own 5V and GND pins?
Note: I have read this question, in which the answers suggest that just connecting the ground lines would be enough, and this one, which confirms my belief that it's OK for the USB hub to just draw 100mA and not negotiate for its current requirements.