問87
頻出度★☆☆
USB は, キーボード, マウスなど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 の機能にないものはどれか。
平成23年度 特別試験 ITパスポート試験 問87
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 87
Incorrectア
Bus power is a feature of USB, so choosing it is wrong. Besides signal lines, a USB cable also contains power lines, so the PC side (the host) can supply 5 V to the connected device. A device that runs on this supply is called bus-powered; mice, keyboards, USB flash drives and small external drives can be used without an AC adapter. The term pairs with self-powered, where a device with high power consumption has its own power supply.
- イ
正解 The correct answer is イ, parallel transfer. USB stands for Universal Serial Bus, and as the word Serial in the name shows, it is an interface that uses serial transfer: one pair of signal lines carries the data one bit at a time, in sequence. Parallel transfer, by contrast, bundles several signal lines together and sends several bits at the same time; it was used by the old printer parallel port (Centronics), by IDE/ATA for internal disks, and by SCSI. In theory parallel transfer can send many bits at once, but as speeds rise, skew (differences in the arrival time of the signal on each line), noise and crosstalk become problems, and the cable becomes thick and expensive. USB therefore adopted serial transfer from the start, concentrating the transfer into a single thin cable by using differential signalling over one pair of signal lines, and achieved higher speed by raising the transfer rate itself (up to 480 Mbit/s with High-Speed in USB 2.0). In other words, parallel transfer is the older method that USB replaced, and it is not a feature of USB 2.0. The remaining ア, ウ and エ are all features that USB 2.0 provides as standard, so the odd one out to choose is イ.
Incorrectウ
Plug and Play is a signature feature of USB, so choosing it is wrong. When a device is connected, the OS automatically reads the device type and identification information and automatically installs or assigns the matching device driver, so the user can start using it with almost no setup work. USB is a standard that was designed on the premise of Plug and Play.
Incorrectエ
Hot plug (live insertion and removal) is also a feature of USB, so choosing it is wrong. It is the mechanism that lets you plug and unplug a device while the PC is powered on and running: as when using a USB flash drive, you can connect and remove devices without shutting the PC down each time. Note that when removing a device, the rule is to first carry out the removal operation in the OS (safely remove the hardware) so that data still being written is not lost, and only then unplug it.
ここがポイント
- USB stands for Universal Serial Bus and uses serial transfer, sending data one bit at a time over a single pair of signal lines. It is the opposite of parallel transfer, which sends several bits at once over several signal lines (the old printer port, IDE/ATA, SCSI and so on), so parallel transfer is not a feature of USB.
- USB 2.0 provides three representative features: bus power (supplying power from the power lines in the cable), Plug and Play (automatic recognition and automatic driver setup on connection), and hot plug, that is, live insertion and removal (plugging and unplugging while the power is on). It is a good idea to memorise these three together with serial transfer as a set.
用語