Does the Single need a powered USB hub? If not, is there a limit of the number of unpowered devices that can be hooked up to a single USB port on the motherboard?
No, it has an external power brick. But a powered hub often tends to be higher quality anyway, and you might not be able to get an unpowered version with a large number of ports. The limit is 100 BFL Singles per system. The USB limit is 128, including the hubs themselves.
Actually it's 127, not 128, and hubs with more than four ports are often two cascaded 4 port hubs internally. The tree may only be 8 hops (including those "internal" hubs and the device) deep.
A port of a USB host, or a self-powered (as in has a power brick, not powered by the bus) hub, can deliver up to 500mA, if the specs are met. (Some cheap self-powered hubs actually can't provide that on all ports at the same time, but usually they at least claim to.)
Each hub will allocate 100mA for itself, and each port needs to be allocated a minimum of 100mA in order to detect devices.
So in theory, you can't cascade bus-powered hubs by definition. However in many cases these constraints are completely ignored by manufacturers, allowing some combinations to work that should not be allowed in theory.
All of the currently available FPGA boards should only draw a maximum of 100mA, so they should be happy on every USB port, given the obvious constraints are met:
- Maximum of 127 devices total per bus, counting hubs as well, including internally cascaded hubs
- Cascade hubs at maximum 7 levels deep, also counting internally cascaded hubs
- Only the last level of hubs (with no more than 4 ports each) may be bus-powered
Be aware that there might already be some hubs internally to your PC between the "root ports" and the actual ports on the PC's case, and that multiple USB "root ports" might be sharing the same bus and thus 127 device limit. Things like internal card readers, wifi/usb dongles, ... might also take away addresses.
