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Various tape drives use optical sensing of the tape reel position to detect the last-wrap condition.
English (United States)
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Microprocessor Last Wrap Sensor
Various tape drives use optical sensing of the tape reel position to detect the
In order to eliminate the cost, inaccuracies, and failure potential of the optical
last-wrap sensor, a technique is described which can be replaced by the sensor
function in the microprocessor. The drive's microprocessor uses the tachometer
sensors on the reel motors, along with a built-in tolerance of the number of wraps
on the supply reel after the so-called physical end of tape, to always determine
accurately the positioning of the tape.
The technique involves calculating relative radii of the reels by a small
forward motion. The calculation is an arithmetic/logical operation performed by
the microprocessor using the two-motor tachometers. The relative radii
determination in turn informs the microprocessor of the amount of tape on each
reel. Thus, beginning of tape logical end of tape, and "physical" end of tape may
be discerned without the aid of an external, problem-fraught optical sensor. This
determination of positioning may be accomplished even after power has been
lost and then restored to the drive (which would eliminate within the drive
electronics the prior record of positioning).