for logical high level, and termination of 45 Ω to ground or 90 Ω differential to match the data cable impedance. To recognize battery charging mode, a dedicated charging port places a resistance not exceeding 200 Ω across the D+ and D− terminals. The VBUS supply from a low-powered hub port may drop to 4.40 V. The USB 3.0 specification introduced an A-to-A cross-over cable without power for connecting two PCs.
If the transfer is from the host to the endpoint, the host sends an OUT packet with the desired device address and endpoint number. If the data transfer is from the device to the host, the host sends an IN packet instead.
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A single device function composed of independently controlled interfaces is called a composite device. A composite device only has a single device address because the host only assigns a device address to a function. When a host starts a data transfer, it sends a TOKEN packet containing an endpoint specified with a tuple of .
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Despite USB 3.0 being 10x faster than USB 2.0, USB 3.0 transfer cables are only 2 – 3x faster given their design. The different A and B plugs prevent accidentally connecting two power sources.
Otherwise, it is accepted and the data transaction can start. A bi-directional endpoint, on the other hand, accepts both IN and OUT packets.
Synchronous – The device’s clock is synced to the USB start-of-frame or Bus Interval signals. For instance, this can require syncing an 11.2896 MHz clock to a 1 kHz SOF signal, a large frequency multiplication. Asynchronous – The ADC or DAC are not synced to the host computer’s clock at all, operating off a free-running clock local to the device. Endpoints are grouped into interfaces and each interface is associated with a single device function. An exception to this is endpoint zero, which is used for device configuration and is not associated with any interface.
These are not meant for data transfer but are aimed at diagnostic uses. Many USB bridge / data transfer cables are still USB 2.0, but there are also a number of USB 3.0 transfer cables.