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Not all USB ports are powered. It is often necessary to connect certain peripherals directly to a USB 2.0 port on the computer (as opposed to, say, a USB jack
by makecheck 17y ago
Not all USB ports are powered. It is often necessary to connect certain peripherals directly to a USB 2.0 port on the computer (as opposed to, say, a USB jack on the keyboard itself).
- stcredzero 17y agoNot only that, but I suspect that the iPad needs a lot of power from a USB port. If the port can't provide enough power, I think the iPad doesn't draw enough to charge itself. I thought that there were power negotiation protocols built into USB for this purpose.
- seattleslow 17y agoEach USB device has a set of descriptors to describe itself and its supported capabilties. The amount of current required by a device (from the cable) is provided in the configuration descriptor. For example, a device could have two configurations that are exactly the same, only differing by the amount of currented needed from the cable. Host software can determine if the port to which a device is connected can provide 500 ma, and if so, select that configuration which instructs that device that it may draw that amount of current.
- NateLawson 17y agoThat is incorrect. All USB ports are powered, it's just the amount of power available varies. The USB spec says all ports must provide 5V. The second part of your comment is correct.
- makecheck 17y agoMy mistake. There are USB hubs that are classified as "powered" or "unpowered", but I guess in the unpowered case it's relying on the computer's supply.