Method for Handling Interlayer Slip
Original Publication Date: 1999-Dec-01
Included in the Prior Art Database: 2003-Jun-11
The stacking of a spool of magnetic tape is effected by several factors. The environment in which the media is stored, and the length of time it is stored will cause changes in the stack. Variation in temperature and humidity cause mechanical stresses in the media which effect the tape tension and interlayer pressure. Wear and physical stresses on the media will also effect stacking. The end result is interlayer slip or "loose wraps". In a tape drive, interlayer slip can pose many problems. One significant problem is data integrity. During the write process, the instantaneous change in velocity that accompanies an interlayer slip will cause the data to be written at the wrong density. The read back check performed as the data is written cannot detect the change in density since the velocity is unchanged. If the change in density exceeds the capability of the read channel, the data will be unreadable on the same drive or any other drive. Therefore, it is very important to detect interlayer slip and recover from it. Another significant problem associated with interlayer slip is physical tape damage. Interlayer slip causes shifting in the stack which can damage the media. The media used the 3590 tape drive today is much thinner and is more susceptible to damage than the thicker stronger media used in 3480 and 3490. If the media is compressed and wrinkled by the slip, any data in that area may be unreadable. Detecting interlayer slip is essential to prevent data loss and tape damage. The method of detecting interlayer slip on the thicker media of the 3480 and 3490 tape drives (IBM Patent #US4389600: Tape media interlayer tension check), is not suitable for the thinner media used today in the 3590. The old method accelerated the tape at a higher than nominal tension and looked for errors resulting from the slip. At that point, the tape was refreshed at high speed. The thinner media used today would be damaged by this test. A new method has been developed that depends on tension and velocity feedback to detect interlayer slip.