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USB On-The-Go, often abbreviated to USB OTG or just OTG, is a specification first used in late 2001 that allows USB devices, such as digital audio players or mobile phones, to act as a host, allowing other USB devices, such as USB flash drives, digital cameras, mice, or keyboards, to be attached to them. Use of USB On-The-Go allows those devices to switch back and forth between the roles of host and client. For instance, 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. In other words, USB On-The-Go 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 On-The-Go device, they establish a communication link. The device controlling the link is called the master or host, while the other is called the slave or peripheral. == Overview == Standard USB uses a master/slave architecture; a ''host'' acts as master device for the entire bus, and a USB ''device'' acts as slave. Devices are designed from the outset to assume one role or the other computers are generally set up to be hosts, while, for example, printers are normally slaves. When a device is plugged into the USB bus, the master device, or host, sets up communications with the device and handles service provisioning (the host's software enables or does the needed data handling such as file managing or other desired kind of data communication or function). The host is responsible for all data transfers over the bus, with the devices capable of only signalling (when polled) that they require attention. To transfer data between two devices, for example from a phone to a printer, the host first reads the data from one device, then writes it to the other. That allows the devices to be greatly simplified compared to the host; for example, a mouse contains very little logic and relies on the host to do almost all of the work. While the master/slave arrangement works for some devices, there are many devices capable of acting as either master or slave depending on what else shares the bus. For instance, a computer printer is normally a slave device, but when a USB flash drive containing images is plugged into the printer's USB port with no computer present (or at least turned off), it would be useful for the printer to take on the role of host, allowing it to communicate with the flash drive directly and print images from it. USB On-The-Go recognizes that a device can perform both master and slave roles, and so subtly changes the terminology. With OTG, a "device" can be either a "host" when acting as a link master, or a "peripheral" when acting as a link slave. The choice between host and peripheral roles is handled entirely by which end of the cable the device is connected to. The device connected to the "A" end of the cable at start-up, known as the "A-device", acts as the default host, while the "B" end acts as the default peripheral, known as the "B-device". After initial startup, setup for the bus operates as it does with the normal USB standard, with the A-device setting up the B-device and managing all communications. However, when the same A-device is plugged into another USB system or a dedicated host becomes available, it can become a slave. USB On-The-Go does not preclude using a USB hub, but it describes host/peripheral role swapping only for the case of a one-to-one connection where two OTG devices are directly connected. Role swapping does ''not'' work through a standard hub, as one device will act as a host and the other as a peripheral until they are disconnected. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「USB On-The-Go」の詳細全文を読む スポンサード リンク
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