Magnetic Tape

Magnetic tape memories look like an audio tape, where the tape itself is a thin strip of plastic coated with magnetic recording material. It is the most commonly used sequential access secondary storage device. It is available in the form of cassettes, Reels and cartridges with reels being most popular of the 3 packages. Reels used with minis and main frames have half an inch wide tape and the tape length per reel is about 2400 feet. It can store 1600 characters per inch of the tape length. Information contained on one page could be stored on just an inch and a quarter of the tape length and about 50 books could be stored on one reel of the tape.

These reels are run on machines called tape drive units. It has a supply reel, a take up reel, a Read/Write head and an erase head. In case there is some recorded data on the tape and a fresh data is written, previously recorded data is erased by the erase head and the new data is recorded.

The process is similar to the one we all are familiar with in case of audio tape or audio cassette recorders.

Data on magnetic tape is represented by tiny magnetized spots with the presence of a spot representing a ‘1’ and the absence a ‘0’. A simple and commonly used method of representing data is by using EBCDIC code. The tape width can be divided into 9 bits to represent one character. The parity bit has been chosen to have an odd parity. The bit reference numbers and the parity bit position are also indicated. Reference number 0 indicates the position of the first EBCDIC bit.

Data is often stored in terms of records with individual records separated by a gap. Also when records are short, they may be grouped to form blocks. The data stored in the form of blocks will then have inter-block spacing.

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