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Magnetic Tape Error Recovery Subsystem

IP.com Disclosure Number: IPCOM000082763D
Original Publication Date: 1975-Feb-01
Included in the Prior Art Database: 2005-Feb-28
Document File: 3 page(s) / 54K

Publishing Venue

IBM

Related People

Iskiyan, JL: AUTHOR

Abstract

Disk data (count, key, and data format) is contiguously recorded on a series of slant tracks on a magnetic medium by a helical scan recorder. Signal transfers occur between the tape recorder and disk recorder and between the disk recorder and a CPU. Since all of the data on the tape recorder is in disk format, no translation is required in the tape-to-disk transfers.

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Magnetic Tape Error Recovery Subsystem

Disk data (count, key, and data format) is contiguously recorded on a series of slant tracks on a magnetic medium by a helical scan recorder. Signal transfers occur between the tape recorder and disk recorder and between the disk recorder and a CPU. Since all of the data on the tape recorder is in disk format, no translation is required in the tape-to-disk transfers.

In recording and readback from the flexible magnetic medium, such as magnetic tape, errors occur causing so-called "permanent" read errors. When such errors do occur, the data signals recorded between the error locations, such as E1 and E2, and the end of a tape track may not be completely recoverable.

In a helical scan recorder, the direction of scan of the respective tracks is in the direction of the arrow; there is no backward read to achieve data recovery between the trailing end of the track (bottom of the tape) and the error position. While phase and clocking resynchronization may be achieved between the error location and the trailing end of the track, the logical partitioning of the recorded data is usually not identifiable.

Enhanced error recovery in such a recorder is achieved, by providing a table track in association with each recorded cylinder of disk data recorded on the tape. The information content of the table track identifies, in a logical fashion, the cylinder and disk track number with the physical location of such disk data on the magnetic tape. The tape location of the beginning of each disk track is identified by the tape track number, plus an offset in bytes. For example, for cylinder 01, track 00 begins at track 00 with a 0000 offset as indicated by X0 on the magnetic medium diagram.

Track 01 of the same cylinder happens to begin at track 01 of the magnetic medium also with a 0000 offset and identified by the symbol X1. Track 02 of the same cylinder begins in track 03 of the magnetic tape at offset 1241 bytes. Disk track 01 extends from magnetic medium track 01 over tracks 02 and 03. The remainder of the disk data mapping in the cylinder area of the magnetic medium follows the above description for all tracks within a given cylinder. Additionally, the table track has a list of bad tape tracks. In the example, tracks 10 and 21 of the tracks assigned to cylinder 01 are bad, i.e., cannot receive recorded signals.

To recover from error E1, reference to the table track is necessary because it is not known where disk track 01 begins. It so happens that the disk track begins coincidently at the beginning or leading edge of tape track 01. Accordingly, error El does not impact the reading of disk track 01. Similarly, error E2 in track 03 impacts disk track X2 and disk track X3, since both of those disk tracks have their beginning in tape track 0...