問70
頻出度★☆☆
USB は, PC にハードディスク, プリンタなどの様々な周辺機器を接続できるインタフェースである。USB 2.0 に関する記述のうち, 適切なものはどれか。
平成22年度 秋期 ITパスポート試験 問70
テクノロジ系コンピュータ構成要素 › 入出力デバイス
This question's explanation and your answers so far.
解説
問 70
Incorrectア
The connector shape is not unified into a single type, so this is wrong. In USB 2.0, at least two shapes are used separately so that the connecting side and the connected side are not mixed up: the flat rectangular Type A used on the port side of a PC or a USB hub, and the square Type B used on the peripheral-device side, such as a printer. In addition, smaller shapes such as mini-B and micro-B are defined for portable devices and digital cameras, so in practice several shapes coexist. Because the shapes are separated, two host sides or two device sides cannot be connected together by mistake, and the direction of power supply (from the host to the peripheral device) is naturally determined as well.
Incorrectイ
The data transfer rate cannot be chosen and set by the PC user, so this is wrong. In USB, the moment a device is plugged in, the host (the PC) and the peripheral device negotiate automatically, and the fastest mode that both sides can handle is selected. USB 2.0 defines three modes: Low Speed at 1.5M bits per second, Full Speed at 12M bits per second, and High Speed at 480M bits per second. For example, a mouse uses Low Speed and a USB flash drive uses High Speed; the mode is determined by what the device side supports. There is no operation in which the user picks a speed from a settings screen. Note that this mechanism, in which a device is recognized and configured automatically just by plugging it in, is called plug and play.
- ウ
正解 The correct answer is “ウ A peripheral device with low power consumption can obtain power supply simply by connecting via USB, without being connected to a power source.” A USB cable is built by bundling together not only the signal lines that carry data but also power lines (+5V and GND). Because of this, a port on the PC or on a USB hub can supply power at the same time as it sends data to a peripheral device. This mechanism is called bus power, and USB 2.0 specifies that up to 500 milliamperes (about 2.5 watts at 5V) can be supplied per port. Devices whose power consumption stays within this range, such as a mouse, a keyboard, a USB flash drive, a web camera, or a small external drive, operate just by plugging in a single USB cable, without being connected to an AC adapter or a wall outlet. Conversely, devices that exceed this upper limit, such as printers and large external hard disks, use the self power method with their own power supply and need a separate AC adapter. The question attaches the condition “a peripheral device with low power consumption” precisely because it refers to devices that bus power can cover, so this statement follows the USB 2.0 specification and is appropriate.
Incorrectエ
This choice is wrong on two counts. First, USB stands for Universal Serial Bus; it is a serial interface that sends data one bit at a time in order over a single signal line, and it is not a parallel interface. Second, when several peripheral devices are connected through a USB hub or the like, they share the bandwidth of one bus (a common transmission path). Therefore, if several devices exchange large amounts of data at the same time, the effective transfer rate per device naturally drops. The conclusion that the speed does not slow down does not hold either.
ここがポイント
- Remember USB bus power: a USB cable carries both signal lines and power lines, so a peripheral device with low power consumption (a mouse, a keyboard, a USB flash drive, and so on) works with no AC adapter, receiving its power from the USB cable alone. The supply limit of USB 2.0 is about 5V and 500 milliamperes per port; printers and large external hard disks that exceed this use the self power method with their own power supply.
- Organize the basic properties of USB by term: (1) As the name says, it is a serial interface, and the connected devices share the bandwidth of the bus, so the speed per device drops when they are used at the same time. (2) On connection the host and the device negotiate automatically to decide the transfer mode (1.5M / 12M / 480M bits per second in USB 2.0); the user does not set it. (3) It supports plug and play, where a device is recognized and configured just by plugging it in, and hot plugging, where a device can be inserted or removed while the power is on. (4) Connectors come in several shapes such as Type A, Type B, mini-B, and micro-B.
用語