Tape positioning using reverse boundary capture: tape drive system and method

DETAILED DESCRIPTION A tape drive system and method provides for increased precision during repositioning by using block boundary captures made while the a is moving in a rearward direction.
This generally allows the errors due to at least one tape direction reversal to be cancelled in a repositioning procedure.
In order to perform a hitch, the rear boundary of a block can be captured while the tape is moving in reverse.
This "reverse capture" cancels the effect of prior errors, provided they are within tolerance.
Thus, when forward motion is resumed, the position error is fairly restricted to that incurred during the next reversal to forward movement, without the confounding effect of prior reversals.
In a backspace operation, a front boundary gap can be captured while the tape is moving in reverse.
The value of the reverse-captured front boundary can be used as the reference for repositioning, cancelling errors due to prior reversals in direction.
In one implementation of the present invention, a tape drive system includes means for capturing front and rear boundaries while the tape is moving forward and while it is moving backwards.
These captures can be performed in two independent ways within a single system.
In a gap transition approach, transitions in a signal indicating whether a block or gap is present can be used to identify block boundaries.
In a boundary marker approach, conventional preambles and postambles can be read and identified in forward or reverse.
A repositioning strategy must address the challenge of ensuring that a captured boundary belongs to the correct block.
It is possible for a block to be followed by a very short block which is crossed while the system is ramping down to a stop.
Without precautions, the rear boundary of the second block could be captured and mistaken for the rear boundary of the intended block.
Accordingly, during the forward motion of the tape, the rear boundary of the subject block can be captured, effectively cancelling accumulated position errors