A tape drive or streamer (English ) is a device for storing data on magnetic tape. In the early days of computer technology in the 1950s and 1960s, the large band stations, each with two standing side by side mounted reels the epitome of the appearance of the computer itself, at least in the world of cartoons and commercial art were.
The Datasette was in an intermediate phase in the 1970s and 1980s, a widespread drive for home computers. Today's tape drives are designed for professional users, and special types of magnetic tape cartridges and are generally used for regular data backup medium to large amounts of data.
Principle of operation
As cassette recorders have tape drives of a drive for two coils, one Aufnahme-/Wiedergabekopf and an erase head. The data is serially written or read. The available storage capacity depends not only on the length of tape. It also depends on the standard used (AIT, QIC, DAT, DCC, DDS, SLR, DLT, FSK, LTO, VXA, etc.) and extends up into the terabyte range. For enclosures with only one reel is called a cartridge (as opposed to cassette that has two coils ).
- Linear recording: The magnetic tape is described in the forward and backward direction with a plurality of adjacent longitudinal tracks.
- Helical scanning ( helical scan ): The magnetic tape is skewed on the ( rotating ) write head over.
- Tapering recording: The magnetic tape is first described very fast, later, the recording takes longer.
- Motorway recording: The wide magnetic tape ( such as tape ) with several parallel and simultaneously generated data tracks described, for example, with 7 or 11 tracks. This led to a variety of formats, making it difficult in retrospect hard to read older tapes correctly.
There are two methods to write data to a magnetic tape, the start-stop method and the streaming process (streaming mode).
The start-stop method, individual data blocks are always written to the tape. Because when writing the tape at a certain minimum speed must pass by the write head, the tape for each write operation ( start-up ) is initially accelerate. After writing the tape is stopped again. Because of these start and stop phases must be provided on the magnetic tape between the data sets empty areas. These are called in the jargon clefts or lnterblock gaps.
This procedure is only very rarely used.
A process described magnetic tape therefore looks like this:
XX .......... .......... XX XX XX .......... .......... where XX is the user data and "...." the gaps.
This shows that a tape written is to a large extent from unused dens and only a small portion of user data. In order to increase the amount of useful data, you can see the data block. They are surrounded by one more ( short ) logical records in the main memory of the computer to a ( long ) physical data and writes it to the tape. This has several advantages: The tape is better exploited because increases the payload data. Also, the belt unit and the magnetic tape can be protected, because fewer write accesses and thus the significant mechanical stress due to the starting and stopping is reduced. This is called the length of the original logical record length records, the length of the blocked set physical record length and the number of logical records in a physical record blocking factor.
Example: If you want to transfer the data from punch cards to tape, then you have a logical record length of 80 characters. Blocks are 100 punch cards to a physical set, then the physical record length is 8000 characters and the blocking factor is 100 It is usually advantageous to set up a big blocking factor. Modern devices ( DLT, LTO, etc.) work with a block size of 64 kilobytes, and more.
The streaming mode is the most advanced recording method. In streaming mode, the tape is ( without stopping ) continuously described. This requires that the data be supplied fast enough (e.g., from a buffer memory ) in the magnetic tape unit. This procedure is much faster than the start-stop method, band and tape drive are protected. Even so are fewer to no empty gaps. Must still be stopped, so called File Marks are written to the tape. The restart after a stop, for example, in the modern DLT and DLT - S drives with virtually no loss, but you have when recovering the tape to rewind a bit. This start -stop mode (also called Shoeshine effect) not only leads to a dramatic drop in the write rate, but also to greatly increased wear on magnetic tape ( by the frequent stops is it more stretched) and drive. Therefore it must be ensured especially with modern drives with their high data transfer rates to a well-filled data buffer.
Tape drives to be installed in computers have a front panel with an eject button and some LEDs that indicate the status. They are "built - shafts. Besides conventional drives with the full height ( 3 ½", 5 ¼ in ) of a shaft also drives the half-height (1 ¾ " ) are available, which are therefore not larger than an optical drive.
Tape drives with their own housings also come with its own power supply. They can be used when there is no installation space is available in the computer. In addition, they can be taken easily to other computers in operation.
Tape drives for mounting in tape libraries are tailored to their type and not installed in the rule in other types.