I'm using a Microchip USB2517 7-port USB hub in a self-powered application, and I'm interested in the behaviour of the device if the upstream VBUS detect signal is removed. I'd like to know if the downstream VBUS is disabled when upstream VBUS is removed. The datasheet mentions that power to the differential pins must be removed when upstream VBUS is no longer detected. The datasheet isn't clear about the port power signal, however.
My reading of the USB 2.0 standard gives me an answer that's open to interpretation. From section 11.5, it's clear that the port enters the powered-off state when external power to the hub is removed. 126.96.36.199 says "If a hub was configured while the hub was self-powered, and then if external power is lost, the hub must place all ports in the Powered-off state." Unfortunately, it's not clear whether "external power" refers to the upstream VBUS detect signal being de-asserted, or whether it refers to the IC's external power (Vcc). Under the former interpretation, if there is no upstream devices the hub will not supply power to any downstream devices. Under the latter, the device will still supply some power (also not clear. One unit load? The negotiated number of unit loads? Without power on the differential pair, the device obviously can't request more than one unit load if plugged in after the fact.)
TL;DR: Is there any power available on the downstream ports of a self-powered hub if the upstream port isn't connected?
Related: Does USB Host always supply power on VBUS? (doesn't describe behaviour during powered-off state)