The OTG A-device is a power supplier, and an OTG B-device is a power consumer.

USB OTG defines two roles for devices: OTG A-device and OTG B-device, specifying which side supplies power to the link, and which initially is the host. The device controlling the link is called the Host, while the other is called the Peripheral. USB OTG introduces the concept of a device performing both Host and Peripheral roles – whenever two USB devices are connected and one of them is a USB OTG device, they establish a communication link. A mobile phone may read from removable media as the host device, but present itself as a USB Mass Storage Device when connected to a host computer. Use of USB OTG allows those devices to switch back and forth between the roles of host and device.

USB On-The-Go ( USB OTG or just OTG) is a specification first used in late 2001 that allows USB devices, such as tablets or smartphones, to act as a host, allowing other USB devices, such as USB flash drives, digital cameras, mouse or keyboards, to be attached to them. USB On-The-Go adapter for USB-B Micro charging ports of smartphones and tablet computers without dedicated USB-A port

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