External Computer Devices
External, or peripheral, computer devices are divided into devices for input and output of information and external storage devices (Figure 4.3).
Fig. 4.3. Classification of external computer devices
External Storage Devices
External storage devices (VCD) are magnetic, optical, magneto-optical and electronic principle of operation. A storage device is a device for writing and/or reading information from a media of a certain type.
In magnetic media drives , the magnetic principle of recording is used, in which two different magnetization states corresponding to a record of zero and one can be fixed on a surface covered with a layer of magnetic material. Having appeared in the 1960s, magnetic disk drives are still the most necessary for the operation of a computer system. This is due to the presence of such properties as high speed of recording and reading information, high density of information recording (number of bits per unit of media area), the possibility of repeated use. The disadvantages include low reliability: the recorded information is subject to the influence of electromagnetic fields, as well as to mechanical influences.
There are magnetic VDS with serial and direct access to information. A hard drive with sequential access includes tape drives called streamers. The main drawback of these is the low write/reading information, typical for devices with serial access (to access the necessary information block, it is necessary to sequentially review all the previous sections of the media). The advantages include high capacity (hundreds and thousands of gigabytes), low cost (this is the cheapest way to store data in terms of megabytes), stability. Streamers are used to snap archival data and backup.
In magnetic VDS with direct access to information, data access is performed directly at the address of the information, therefore, high speed of data writing/reading can be provided. The carrier of information is hard or flexible magnetic disks (floppy disk drives are obsolete and little used).
Hard disk drive (HDD) - the main device for storing large amounts of information, which is one or more disks protected by a hard case. In desktop PCs, the form factor 3.5 HDD are located inside the system unit (in notebooks form factor 2.5 ").
Often you can hear how instead of the term hard disk use the concept of "Winchester", although the computer memory device does not at all resemble a gun. The origin of the familiar term is explained by the analogy of the short description of the first hard disk "30/30", created in 1973, containing 30 tracks of 30 sectors each, and 30/30 rifles. The capacity of the first disk was 16 KB (imagine how small?).
Now the capacity of hard drives reaches hundreds and thousands of gigabytes. The speed of the HDD is characterized by the read/write speed and the average access time. In general, the speed of the disk is determined to the greatest extent by the speed of rotation of the plates, which reaches 10 000-15 000 rpm and more.
Internal HDDs are connected via SATA interfaces (replacing IDE and EIDE) and SAS (replacing SCSI).
In addition to internal HDDs, external hard disk drives can be used that connect via USB or FireWire interfaces.
RAID array - these are several interchangeable magnetic disks in a single rack. Recording different blocks of a single file can go in parallel to several disks. In addition, the same data can simultaneously be written to multiple disks (mirroring) to improve the reliability of data storage. There are various options for combining parallel recording and mirroring. RAID arrays are used in servers in cases where it is necessary to parallel write/read large data streams for multiple users and ensure high reliability of their storage.
In optical storage , when information is recorded under the influence of optical radiation, the state of the individual regions of the media changes, as a result, when reading in these areas, the laser beam will be reflected or absorbed, which is interpreted as binary zeros or ones. Appearing at the end of the 20th century, optical drives have firmly taken their position among external storage devices, and now no computer is released without them. This can be explained by such advantages of optical media as large capacity, low unit cost of information storage, independence from electromagnetic fields.
CD-ROMs (Compact Discs) use removable media that are divided into CD-ROMs only for reading CD-ROM information (Read Only Memory), CD-R (Recordable) discs and disks with multiple recording of information CD-RW (Rewritable). A regular CD contains 700 MB of information.
In 1996, there appeared DVDs (Digital Versatile Disc) that looked like CDs, but due to changes in physical characteristics and application of new technologies, they allowed storing much more information (4.7 GB in the first models). Now there are several types of DVD-disks, differing in the number of layers on the media, with disks being one-sided and two-sided. For example, DVD-9 - a one-sided two-layer disk with a capacity of 8.5 GB; DVD-18 - a two-layer two-sided disk with a capacity of up to 17 GB.
Like CDs, DVDs can only be used for reading information, with single and multiple recording of information. But unlike CD, several different standards were developed: DVD-R, DVD-RW, DVD + RW, DVD-RW, DL-RW, DVD-RAM. Now, when the prices for optical drives have significantly decreased, many computers are equipped with an optical disk drive that is capable of supporting work with several formats. For example, recording: CD: R24x/RW16x, DVD: R16x, RW8x, RAM 12x means that the drive can write and read information from CD-R, CD-RW, DVD-R, DVD-RW DVD-RAM. The speed of reading/writing optical disks is measured in conventional units, the multiplier "1x" is 150 KB/s. Therefore, the expressions 24x & quot ;, 16x etc. mean that the drive's speed is 24 • 150 Kbytes/s, 16 • 150 Kbytes/s, etc., respectively. It should be borne in mind that the recording and reading speeds from the same type of media are usually different, but the playback speed of audio and video data remains single, and the characteristics of the drive are reflected only on the quality of information reproduction.
In 2006, another standard of optical media appeared: BD (Blu-ray Disc - from English blue ray - blue ray). By using a laser with a shorter wavelength (blue-violet, while the DVD uses red, in CD-infrared lasers), a higher recording density is achieved, resulting in a large capacity (up to 50 GB) of BD media. The drives on BD-disks are issued only for reading (BD-ROM), for write-once (BD-R) and for multiple writing (BD-RE), and also allow recording and reading of all CD and DVD formats.
Storage on optical media can be internal and external. Internal connect via SATA (or IDE) and SCSI interfaces, external - via USB, FireWire or eSATA (external SATA). External drives are slower, but allow you to connect and disconnect without shutting down the computer and rebooting the operating system.
In magnetic-optical storage devices , the technologies of magnetic and optical recording principles are combined: the carrier surface can be magnetized only when it is heated to a high temperature, which is achieved by means of a laser beam. Magnetooptical media due to a higher density of information recording allows you to store on the same area of large amounts of information than magnetic, resistant to external influences, allow almost unlimited number of cycles of rewriting.
In a PC, drives on magneto-optical media are not widely used because of the high cost. They are used mainly for data backup and storage of rarely used information.
Flash Drives - SSD (Solid State Drive) are a solid state flash drive based on flash chips for long-term data storage. Unlike magnetic drives flash drives ns have mechanical elements. The flash drives made on the basis of electronic microcircuits have a number of advantages: fast access time, high reliability, compactness (in shape and size resemble a conventional keychain 5-7 cm in length), low power consumption, ease of use.
Modern flash drives can hold up to 500 GB of information and become competitors of HDD, but while large models have a very high price, as well as a limited number of read-write cycles (about 100,000), which can be significant if you use SSD for storing the operating system.
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