Such phones, as slaves, could not readily be connected to printers as they also implemented the slave role. In the absence of USB OTG, cell phones often implemented slave functionality to allow easy transfer of data to and from computers.

If implementing standard USB, devices must assume one role or the other, with computers generally set up as hosts, while (for example) printers normally function as slaves. Standard USB uses a master/slave architecture a host acts as the master device for the entire bus, and a USB device acts as a slave.

Ī USB OTG setup involving a number of devices The initial role of each device was defined by which mini plug a user inserts into its receptacle. The host and peripheral modes may be exchanged later by using Host Negotiation Protocol (HNP). In the default link configuration, the A-device acts as a USB host with the B-device acting as a USB peripheral. 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 master or host, while the other is called the slave or peripheral. USB OTG introduces the concept of a device performing both master and slave 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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