The use of tape drive systems as nonvolatile data storage for archival purposes, as well as for retrieval for data processing, has become increasingly commonplace. A tape drive system typically interacts with, and interfaces to, a computer or PC or the like through an application specific integrated circuit (ASIC) incorporated within or associated with the computer. FIG. 1 is a simplified illustration of a tape path mapping between take-up and supply reels. The tape 18 passes through an assembly 16 of read and write heads structurally arranged to appropriately configure the tape path between supply reel 32 and take-up reel 34. As indicated by the arrows, the tape can be driven bidirectionally so that each reel can perform both supply and take-up functions.
As can be appreciated, the requirements for accuracy in reading and writing of data on tape are much more critical than in analog applications in which tape recording and playback provide audio or video presentations. While a tape error in the latter systems may result in a momentary glitch in audio or video that may not even be noticed, the density of data bit storage and necessity for negligible bit rate error imposes potential disastrous effects on the data processing capability if a tape error should occur. Accuracy and consistency of the drive rate at which the tape traverses the read and write heads are of ultimate importance. Tape drive systems for data storage and retrieval thus have become highly sophisticated in controlling the motor drive characteristics of the drive motors for each reel motor, in maintaining appropriate tension in the tape path, and in controlling threading of the tape throughout the path to avoid diversion of the tape from its path position or breakage of the tape. These measures ensure accuracy in accessing the appropriate point in the tape during start-up, running and stopping conditions. During normal operations, precision stopping of the tape is accomplished by increasing back tension on the supply reel while reducing forward tension on the take up reel.
Stopping tape movement in a reel to reel application provides unique difficulties during a power loss situation. The implementation of braking by using motor back emf alone, during such a situation, does not necessarily prevent the loss of tape path integrity. Braking by using back emf on the supply reel is ineffective if the momentum of the supply reel is significantly greater than that of the take-up reel. Such an inequality would occur if the radius of the tape accumulated on the supply reel at the time of power interruption is greater than the radius of the tape accumulated on the take-up reel. The torque provided by braking using motor back emf on the supply reel would not then be sufficient to balance the rates of deceleration of both reels. As the take-up reel will be stopped sooner than the supply reel, the appropriate tension along the tape path can not be maintained. After the take-up reel is stopped, the continued rotation of the supply reel causes a tape run out, loosening tension in the tape path and subjecting the tape to destruction.
The need exists to maintain tape path integrity, in the event of power failure, with minimal or no tape becoming loosened, either around the reel which supplies the tape, the reel which takes up the tape, or the tape path from reel to reel, other than an amount which might occur during normal operations.