Abstract:
An apparatus, system, and method are disclosed for detecting the formation of a short between a magnetoresistive (“MR”) head and a head substrate. The apparatus is presented with a logic unit containing a plurality of modules configured to functionally execute the necessary steps of generating a baseline electric potential level between a head substrate and ground, monitoring the level of the electric potential between the head substrate and ground, and detecting the formation of a short circuit between the MR head and the head substrate by detecting a change in the electric potential level monitored by the monitoring module from the baseline level to a predetermined threshold level. Beneficially, such an apparatus, system, and method would reduce read errors on the magnetic tape storage system, the time and resources required to recover from such errors, and allow for preventative measures to obviate contamination short related failures of tape drive systems.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates to magnetic tape drives and more particularly relates to detecting the formation of a short between a magnetoresistive (“MR”) head and a head substrate. 
         [0003]    2. Description of the Related Art 
         [0004]    Prior to the advent of automation technology for linear tape drives, large, open tape reels were manually loaded by operators of tape storage systems. As advances in tape cassette and other technologies were adapted to magnetic tapes drives, large automated tape systems became possible. In such a system, magnetic tape cassettes are automatically selected from a library of tape cassettes, and loaded to and unloaded from magnetic tape drives by a robotic arm or other similar automation technology. 
         [0005]    The use of tape cassettes coincided with the introduction of magnetoresistive (MR) thin film heads, which are fabricated using processes similar to those used for hard disk drive heads. Both tape and hard disk drive heads are fabricated on ceramic wafer substrates. The preferred wafer material is aluminum oxide-titanium carbide, which is very hard and also happens to be electrically conductive. In a tape drive, tape contacts the conductive substrate. To avoid adverse tribological process, the substrate may be biased to a preferred voltage level, generally between 0 and 3 volts. Debris from the tape or the environment may collect on the head and electrically bridge the insulation gap between the conductive wafer substrate and the MR heads. The resulting shorting can disrupt the flow of bias current in the MR heads, which is necessary for their proper functioning. The shorting can also disrupt the substrate voltage biasing, leading to further degradation on the surface of the head. 
         [0006]    Thus, debris or contamination of the MR heads may result in a short circuit between the MR head element and the head substrate. A typical short will result in increased read errors, or complete failure to read data to the tape. Because of the rubbing of tape on the heads, shorting related errors are generally more likely to occur in tape drives than in hard disk drives and other types of data storage devices. 
         [0007]    Current magnetic tape storage systems do not provide a proactive method for identifying substrate-sensor related shorts. Typically, the formation of the short goes unnoticed until either read error rates become excessive or the tape drive fails to read data. Substrate-sensor shorts may be correctable at the time of discovery, but the required head cleaning process is often time consuming and disruptive to normal operation of the tape drive system. 
         [0008]    For example, in an automated tape drive system, a short may be detected by an increased error rate while writing data to a tape cassette. In such an example, the automated system must dedicate resources to remove the magnetic tape cassette from the tape drive, insert a head cleaning cassette, allow the cleaning cassette to clean the MR heads, remove the head cleaning cassette, and replace the magnetic tape cassette. This process is often repeated until data can be accurately read or written to the tape. 
         [0009]    As a consequence of the cumbersome head cleaning process, data reliability can be negatively impacted, time and system resources are wasted, and system down time is not efficiently used. It would be useful to provide a solution for early detection of shorts between MR heads and the head substrate. Such a solution would reduce error rates, increase efficiency, and better utilize system down time. 
         [0010]    From the foregoing discussion, it should be apparent that a need exists for an apparatus, system, and method for detecting the formation of a short between a MR head and a head substrate. Beneficially, such an apparatus, system, and method would reduce read errors on the magnetic tape storage system, the time and resources required to recover from such errors, and allow for preventative measures to obviate short related failures of tape drive systems. 
       SUMMARY OF THE INVENTION 
       [0011]    The present invention has been developed in response to the present state of the art, and in particular, in response to the problems and needs in the art that have not yet been fully solved by currently available magnetic tape drive systems. Accordingly, the present invention has been developed to provide an apparatus, system, and method for detecting the formation of a short between a magnetoresistive head and a head substrate that overcome many or all of the above-discussed shortcomings in the art. 
         [0012]    The apparatus for detecting the formation of a short between a magnetoresistive (“MR”) head and a conductive head substrate is provided with a logic unit containing a plurality of modules configured to functionally execute the necessary steps of generating a baseline electric potential level between a head substrate and ground, monitoring the level of the electric potential between the head substrate and ground, and detecting the formation of a short circuit between the MR head and the head substrate by detecting a change in the electric potential level monitored by the monitoring module from the baseline level to a predetermined threshold level. These modules in the described embodiments include a biasing module, a monitoring module, and a detection module. 
         [0013]    In one embodiment, the biasing module further comprises a substrate biasing module and a head biasing module. The substrate biasing module may comprises a voltage divider circuit configured to generate a predetermined baseline electric potential level. In such an embodiment, a short between the head and substrate appears as a parallel resistance across the substrate voltage divider circuit. The parallel combination is obviously less than the substrate voltage divider resistance and so may result in a reduction of the substrate bias voltage. 
         [0014]    The detection module may detect a short circuit between the MR head and the head substrate by detecting a change in electric potential level between the head substrate and ground resulting from the change in resistance between the head substrate and ground. In a further embodiment, the detection module checks the electric potential level monitored by the monitoring module when a tape cassette is ejected. 
         [0015]    In one embodiment, the apparatus further comprises a cleaning module configured to manage head cleaning in response to the detection module detecting a short circuit. Additionally, the apparatus may comprise a notification module configured to notify a tape drive user when a short circuit is detected. 
         [0016]    A system of the present invention is also presented for detecting the formation of a short between a magnetoresistive head and a head substrate. In one embodiment, the system includes a magnetic tape cassette configured to store data on a magnetic tape. The system may additionally include a magnetic tape drive configured to generate a baseline electric potential level between a head substrate and ground, monitor the level of the electric potential between the head substrate and ground, and detect the formation of a short circuit between the MR head and the head substrate by detecting a change in the electric potential level monitored by the monitoring module from the baseline level to a predetermined threshold level. Additionally, the system may include a head cleaning device configured to clean the MR head and head substrate on the magnetic tape drive. 
         [0017]    The system may be further configured to automatically clean the heads on the magnetic tape drive with the head cleaning device in response to detection of a short circuit between the MR head and the head substrate. In another embodiment, the magnetic tape drive may be further configured to notify a tape drive user to load the head cleaning device when a short circuit is detected. 
         [0018]    A method of the present invention is also presented for detecting the formation of a short between a magnetoresistive head and a head substrate. The method in the disclosed embodiments substantially includes the steps necessary to carry out the functions presented above with respect to the operation of the described apparatus and system. In one embodiment, the method includes generating a baseline electric potential level between a head substrate and ground, monitoring the level of the electric potential between the head substrate and ground, and detecting the formation of a short circuit between the MR head and the head substrate by detecting a change in the electric potential level monitored by the monitoring module from the baseline level to a predetermined threshold level. 
         [0019]    Reference throughout this specification to features, advantages, or similar language does not imply that all of the features and advantages that may be realized with the present invention should be or are in any single embodiment of the invention. Rather, language referring to the features and advantages is understood to mean that a specific feature, advantage, or characteristic described in connection with an embodiment is included in at least one embodiment of the present invention. Thus, discussion of the features and advantages, and similar language, throughout this specification may, but do not necessarily, refer to the same embodiment. 
         [0020]    Furthermore, the described features, advantages, and characteristics of the invention may be combined in any suitable manner in one or more embodiments. One skilled in the relevant art will recognize that the invention may be practiced without one or more of the specific features or advantages of a particular embodiment. In other instances, additional features and advantages may be recognized in certain embodiments that may not be present in all embodiments of the invention. 
         [0021]    These features and advantages of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    In order that the advantages of the invention will be readily understood, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments that are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings, in which: 
           [0023]      FIG. 1  is a schematic block diagram illustrating one embodiment of a system for detecting the formation of a short between a magnetoresistive (“MR”) head and a head substrate in accordance with the present invention; 
           [0024]      FIG. 2  is a schematic block diagram illustrating one embodiment of an apparatus for detecting the formation of a short between a MR head and a head substrate; 
           [0025]      FIG. 3  is a detailed schematic block diagram illustrating one embodiment of an apparatus for detecting the formation of a short between a MR head and a head substrate; 
           [0026]      FIG. 4  is a schematic layout diagram illustrating one embodiment of the positioning of MR heads on a head substrate; 
           [0027]      FIG. 5A  is a schematic circuit diagram illustrating one embodiment of a biasing circuit for generating a baseline electric potential level between a head substrate and ground; 
           [0028]      FIG. 5B  is a simplified schematic circuit diagram illustrating one embodiment of a biasing circuit for generating a baseline electric potential level between a head substrate and ground; 
           [0029]      FIG. 6  is a schematic flow chart diagram illustration on embodiment of a method for detecting the formation of a short between a MR head and a head substrate; and 
           [0030]      FIG. 7  is a detailed schematic flow chart diagram illustrating one embodiment of a method for detecting the formation of a short between a MR head and a head substrate; 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0031]    Many of the functional units described in this specification have been labeled as modules, in order to more particularly emphasize their implementation independence. For example, a module may be implemented as a hardware circuit comprising custom VLSI circuits or gate arrays, off-the-shelf semiconductors such as logic chips, transistors, or other discrete components. A module may also be implemented in programmable hardware devices such as field programmable gate arrays, programmable array logic, programmable logic devices or the like. 
         [0032]    Modules may also be implemented in software for execution by various types of processors. An identified module of executable code may, for instance, comprise one or more physical or logical blocks of computer instructions which may, for instance, be organized as an object, procedure, or function. Nevertheless, the executables of an identified module need not be physically located together, but may comprise disparate instructions stored in different locations which, when joined logically together, comprise the module and achieve the stated purpose for the module. 
         [0033]    Indeed, a module of executable code may be a single instruction, or many instructions, and may even be distributed over several different code segments, among different programs, and across several memory devices. Similarly, operational data may be identified and illustrated herein within modules, and may be embodied in any suitable form and organized within any suitable type of data structure. The operational data may be collected as a single data set, or may be distributed over different locations including over different storage devices, and may exist, at least partially, merely as electronic signals on a system or network. 
         [0034]    Reference throughout this specification to “one embodiment,” “an embodiment,” or similar language means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment,” “in an embodiment,” and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment. 
         [0035]    Reference to a signal bearing medium may take any form capable of generating a signal, causing a signal to be generated, or causing execution of a program of machine-readable instructions on a digital processing apparatus. A signal bearing medium may be embodied by a transmission line, a compact disk, digital-video disk, a magnetic tape, a Bernoulli drive, a magnetic disk, a punch card, flash memory, integrated circuits, or other digital processing apparatus memory device. 
         [0036]    Furthermore, the described features, structures, or characteristics of the invention may be combined in any suitable manner in one or more embodiments. In the following description, numerous specific details are provided, such as examples of programming, software modules, user selections, network transactions, database queries, database structures, hardware modules, hardware circuits, hardware chips, etc., to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention may be practiced without one or more of the specific details, or with other methods, components, materials, and so forth. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention. 
         [0037]      FIG. 1  depicts a system  100  for detecting the formation of a short between a magnetoresistive head and a head substrate. In one embodiment, the system  100  includes one or more servers  102  connected to a Storage Area Network (“SAN”)  104 . The system  100  also includes a magnetic tape drive  106  connected to the SAN  104 . The system may additionally include one or more magnetic tape cassettes  108  and a head cleaning device  110 . 
         [0038]    In one embodiment, the server  102  is an application or web server. Alternatively, the server  102  may be a data storage server. In certain embodiments, the server  102  may store data on the magnetic tape cassette  108  using the magnetic tape drive  106 . Data may be stored on the magnetic tape cassette  108  as a backup copy of the data stored on a data storage server  102 . 
         [0039]    The storage area network  104  may comprise a network or fabric of data switches, routers, storage devices, and the like. The SAN may employ a variety of networking protocols for data communication, including Transmission Control Protocol (“TCP”), fibre channel, Small Computer System Interface (“SCSI”), and the like. The SAN  104  may be primarily used for trafficking of data for storage on one or more storage devices. In one embodiment, the SAN  104  may traffic backup data between a server  102  and the magnetic tape drive  106 . 
         [0040]    In one embodiment, the magnetic tape drive  106  is configured to generate a baseline electric potential level between a head substrate and ground, monitor the level of the electric potential between the head substrate and ground, and detect the formation of a short circuit between the MR head and the head substrate by detecting a change in the electric potential level monitored by the monitoring module from the baseline level to a predetermined threshold level. The magnetic tape drive  106  may be further configured to manage data read/write operations for storing data on a magnetic tape cassette  108 . In certain embodiments, the magnetic tape drive  106  may include read/write heads. These heads and associated head substrate are discussed in greater detail with reference to  FIG. 4  below. In one embodiment, the magnetic tape drive  106  is an International Business Machines (“IBM”) TotalStorage™ second generation model 3592 tape storage system. 
         [0041]    In one embodiment, the magnetic tape cassette  108  includes a protective case, one or more tape spools, tape guides, and the like. In one embodiment, the magnetic tape cassette  108  is a half inch tape cartridge for use with a model 3592 tape drive  106 . The tape cassette  108  may additionally include writable magnetic tape. The tape guides may position the tape in close proximity to the MR heads of the magnetic tape drive  106 . 
         [0042]    In one embodiment, the head cleaning device  110  is configured to clear the MR heads and head substrate of contaminants. The cleaning device  110  may include brushes, pressurized air, chemical cleaners, and the like. In one embodiment, the cleaning device  110  is an external cartridge that is inserted into the magnetic tape drive  106 . Alternatively, the cleaning device  110  may comprise a brush system, or other cleaning system, stored and operated internally within the magnetic tape drive  106 . 
         [0043]      FIG. 2  illustrates one embodiment of an apparatus  200  for detecting the formation of a short between a MR head and a head substrate is provided with a logic unit containing a plurality of modules configured to functionally execute the necessary steps of generating an electric potential difference between an MR head and a head substrate, monitoring the level of the electric potential difference between the MR head and the head substrate, and detecting the formation of a short circuit between the MR head and the head substrate by detecting a drop in the electric potential difference monitored by the monitoring module to a predetermined threshold level. These modules include a biasing module  202 , a monitoring module  204 , and a detection module  206 . 
         [0044]    In one embodiment, the biasing module  202  comprises a series of electronic circuit components necessary to generate, regulate, and maintain a bias voltage on the head substrate and the MR heads. In such an embodiment, the biasing module may generate a baseline electric potential level between the head substrate and ground. In one embodiment, the biasing module  202  may comprise separate circuits for biasing the MR heads and the head substrate. For example, the MR heads may be substantially grounded, or biased to 0 Volts, and the head substrate may be biased to 1.5 Volts. Alternatively, the head substrate may be biased to from 1 to 5 volts. In another alternative embodiment, the head substrate may be biased to 0 Volts, or grounded, and the MR heads may be biased to a negative potential level. In certain embodiments, the bias module  202  may comprise programmable or adjustable voltage and current sources. One particular embodiment of the bias module  202  is discussed in greater detail with respect to  FIG. 5 . 
         [0045]    In one embodiment, the monitoring module  204  comprises a volt meter connected between the head substrate and ground. The monitoring module  204  may be internal to the magnetic tape drive  106 . Alternatively, the monitoring module  204  may be connected to access pads, but external to the magnetic tape drive  106 . The monitoring module  204  may continuously monitor the potential difference between the head substrate and ground. Alternatively, the monitoring module  204  may monitor the potential difference when triggered or initiated by the detection module  206  or a user. 
         [0046]    In one embodiment, the detection module  206  is configured to detect the formation of a short circuit between the MR head and the head substrate by detecting a change in the electric potential level monitored by the monitoring module  204  from the baseline level to a predetermined threshold level. The value of the predetermined threshold may be about 0.9 Volts. In certain embodiments, a potential difference level of 0.9 Volts indicates an actual short between an MR head and the head substrate. In an alternative embodiment, the threshold may be set to a higher or lower potential difference value to give the user more or less warning that a short is forming. Reasons for the drop in potential difference between the MR heads and the head substrate are discussed further with respect to  FIG. 5 . 
         [0047]      FIG. 3  illustrates another embodiment of an apparatus  300  for detecting the formation of a short between a MR head and a head substrate. In one embodiment, the apparatus  300  includes the biasing module  202 , monitoring module  204 , and the detection module  206  as described with relation to  FIG. 2  above. In one embodiment, the biasing module  202  further comprises a head biasing module  302  and a substrate biasing module  304 . Additionally, the apparatus  300  may include a cleaning module  306  and a notification module  308 . 
         [0048]    In one embodiment, the head biasing module  302  is configured to bias the MR heads. Typically, the head biasing module  302  has a low resistance value with respect to the substrate biasing module  304 . In such an embodiment, a short between the MR head and the head substrate basically shunts the substrate biasing circuit to ground through the biasing module resistors, and this is enough to produce an easily detected change in the substrate voltage. In one embodiment, the head biasing module  302  adds under 1 kOhms of resistance to the path between the MR head and ground. 
         [0049]    In one embodiment, the substrate biasing module  304  is configured to generate a predetermined baseline electric potential level using a substrate voltage reference resistor. The components of the substrate biasing module  304  are described in further detail with respect to  FIG. 5A  and  FIG. 5B  below. In one embodiment, the substrate biasing module  304  has a higher resistance value with respect to the head biasing module  302 . In one embodiment, the biasing module  302  adds 35 kOhms of resistance between the head substrate and ground. 
         [0050]    In one embodiment, the cleaning module  306  is configured to manage head cleaning in response to detecting a short circuit. For example, in an automated tape drive system, the cleaning module  306  may command an automated tape retrieval device to insert a head cleaning device  110  in the magnetic tape deck with the shorted MR heads. Alternatively, the cleaning module  306  may initiate an internal cleaning procedure using internal cleaning brushes, compressed air, or the like. 
         [0051]    In one embodiment, the notification module  308  is configured to notify a user of the tape drive  106  when a short circuit is detected. In a further embodiment, the notification module  308  may notify a user of the tape drive  106  to load the head cleaning device  110  when a short circuit is detected. In certain embodiments, the notification module  308  may display an error message on a user display. Alternatively, the notification module  308  may send a message string, email, page, or other notification to a user. The message may include instructions for cleaning the heads. 
         [0052]    In one embodiment, the cleaning module  306  and the notification module  308  may perform the respective operations in response to a short detected when a tape cassette  108  is ejected from the tape drive  106 . In such an embodiment, the MR heads and the head substrate may be cleaned prior to insertion of another tape cassette  108 . Consequently, disruptions to system operations and read/write operations may be reduced by cleaning the heads offline. 
         [0053]      FIG. 4  illustrates one embodiment of a MR head chip  400  for a MR head reader/writer device on a head substrate  402 . In one embodiment, the chip  400  includes a head substrate  402 , one or more writer heads  404 , one or more reader heads  406 , and one or more servo heads  408 . Additionally, the layout  200  may include a plurality of electrical contact pads  414 ,  416 , and a plurality of electrical connections  410 ,  412 . 
         [0054]    In one embodiment, the head substrate  402  is common to multiple MR heads  404 - 408 . The head substrate  402  may be a conductive AlTiC wafer. Alternatively, conductive silicon may be used. The head substrate  402  may provide structural support and mounting positions for the heads  404 - 408 . The head substrate may be machined to further define the heads  402 - 408 . 
         [0055]    In one configuration, sixteen eight write heads  404 , eight read heads  406  and two servo heads  408  are deposited on the head substrate  402 . In a certain embodiment, the write heads  404  and the read heads  406  are interleaved, and a servo head  408  is positioned on each end of the row of write/read heads  404 ,  406 . The heads  404 - 408  may comprise an insulative layer deposited between the head body and the head substrate  402  for electrical isolation. The insulation layer may not provide complete isolation, but still provide a high degree of electrical isolation. In certain embodiments, the electrical resistance through the insulative layer may be about 10 s of MOhms. 
         [0056]    In one embodiment, the chip  400  includes electrical contacts  414 ,  416  and electrical connections  410 ,  412  between the heads  404 - 408 . A positive connection  410  may be provided to a contact pad  414  designated as a positive contact. A negative, neutral, or ground connection  412  is provided to a negative contact pad  416 . A positive connection  410  and a negative connection  412 , as well as a positive contact pad  414  and a negative contact pad  416  are provided for each MR head on the chip  400 . 
         [0057]    In one embodiment, a short  418  may be formed between an MR head  406 - 408  and the head substrate  402 . The short  418  may be the result of tape material or the product of a reaction between the head and tape. In the depicted embodiment, the contaminant come in contact with both the read head  406  or  408  and the head substrate  402  and bridges the electrical gap originally formed by the insulative layer. In such an embodiment, the impedance between the head  406  or  408  and the substrate  402  is reduced significantly. A typical impedance value may be a few kOhms. The formation of the short  418  may render the read head  406  or  408  inoperable, and result in data read errors or failure of the tape drive  106 . 
         [0058]      FIG. 5A  illustrates one embodiment of biasing circuits  500  for biasing the MR heads  406 - 408  and the head substrate  402 . The circuits may include the head substrate biasing circuit  504  connected to head substrate  502 , and a MR head biasing circuit  506 . In certain embodiments, the head biasing module  302  comprises the MR head biasing circuit  506  and the substrate biasing module  304  comprises the substrate biasing circuit  504 . 
         [0059]    The head substrate biasing circuit  504  may include a DC voltage source  508 , a programmable current source  511  and a ground connection  510 . Additionally, the head substrate biasing circuit  504  may include one or more bypass capacitors  514  and one or more resistors  516 ,  518 ,  519 . In one embodiment the head substrate biasing circuit  504  is configured to apply a bias voltage of 1.5 Volts to the head substrate  502 . The resistors  516 ,  518 ,  519  may be arranged in a voltage divider configuration. For example, the current source  511  may be programmed to supply an 8.57 mA current to the voltage divider circuit. The reference resistor  516  may have a value of 350 Ohms to generate a reference voltage of about 3 Volts. The divider resistors  518 ,  519  may have a resistance value of 35 kOhms each to evenly divide the voltage. In an alternative embodiment, resistor  519  may have a different resistance than divider resistor  518 . In such an embodiment the voltage applied to the head substrate is equivalent to the voltage across resistor  519 . That voltage is determined by the source voltage and the ratio of the resistances of divider resistors  518 ,  519 . A lead extending from between the divider resistors  518  and  519  may be connected to the head substrate  502  resulting in a 1.5 Volt bias potential. The bypass capacitors  514  may be configured to filter transient AC signals. 
         [0060]    In one embodiment the MR head biasing circuit  506  is configured to substantially ground the MR head. The circuit  506  may include a DC voltage source  508 , one or more ground connections  510 , a programmable current source  512 , the MR head, and multiple resistive elements  526 . In one embodiment the MR head has an equivalent resistance  522  of 30-100 Ohms. Modern MR heads are generally provided with magnetic shields  530 , which flank both sides of each MR sensor and which are both magnetically permeable alloys of Ni and Iron or Cobalt or other metals. Because these are metallic, they are usually good conductors. As such, the MR sensors must be electrically isolated from the shields  530  via high resistance insulation. The resistor  530  represents this insulation resistance. However, sometimes the shields are electrically connected to the sensor leads to prevent charge build up on the shields, and in this case the resistor  530  is a thin film device built into the head during wafer processing. The resistance value for resistor  530  is typically 25 to 75 kOhms as mandated by its charge bleeding function. 
         [0061]    Generally, shorting is observed to occur in the gap between the substrate and the shields. This may not act to substantially reduce substrate voltage because the resistors  530  are effectively in series with resistors  526 . However, shorts on multiple sensors may pull the substrate voltage down enough to be detected, as these circuit act in parallel. In addition, additional shorting between the sensor and shields effectively shunts resistor  530  or the insulation resistance when no resistors  530  are used. In this case, substrate shorts in essence connect directly to the sensor through  530 . For the remainder of this discussion, it is assumed that shorting is occurs by either or both of these means and is significant enough to cause a detectable change in the substrate resistance. Ideally, the isolation resistance  524  between the MR head and the head substrate  502  has a value of about 10 s of MOhms. The isolation resistance  524  is not an actual resistor component. The isolation resistance  524  is the equivalent path resistance of the insulative layer. If a short  418  occurs between the MR head and the head substrate, isolation resistance  524  may be reduced substantially. In one embodiment, the isolation resistance  524  may be reduced to less than 1 kOhms. In one embodiment, the resistance drop may reduce the potential level between the head substrate  502  and ground  510  to 0.9 Volts. The resistive elements  526  may have low resistance values, resulting in a near ground potential level on the MR head. 
         [0062]      FIG. 5B  is a simplification of the resistance network between the head substrate  502  and ground  510  created by the substrate biasing circuit  504  and the head biasing circuit  506 . For simplification, only the resistive elements between the head substrate  502  and ground  510  are shown. 
         [0063]    In one embodiment, the substrate voltage reference resistor  519  has a value of 35 kOhms. In such an embodiment, the head biasing resistor  526  has a relatively lower resistance of 130 Ohms. Under normal circumstances, the isolation resistance is about 100 MOhms. In such an embodiment, the resistance between the head substrate  502  and ground through the head biasing circuit  506  is far greater than the resistance to ground through the substrate biasing circuit  504 . Under normal operating conditions, the path between the head substrate  502  and ground  510  through the head biasing circuit  506  may be nearly an open circuit, and consequently not significantly affect the head substrate voltage level. However, differences arise when the head substrate  502  is shorted to the MR heads  406 - 408 . 
         [0064]    In one embodiment, the presence of a short  418  between the MR head  406 - 408  and the head substrate  502  may result in a reduction of the isolation resistance  524  to less than 1 kOhms. In such an embodiment, the total resistance on the head biasing circuit  506  path to ground is reduced to about 1 kOhms. Consequently, the total resistance to ground  510  of the head substrate  502 , which may be defined as the parallel combination of the substrate voltage reference resistor  519  and the isolation resistance  524  in series with the head biasing resistor  526 , is substantially reduced. Consequently, the potential level between the head substrate  502  and ground  510  is noticeably reduced. 
         [0065]    The schematic flow chart diagrams that follow are generally set forth as logical flow chart diagrams. As such, the depicted order and labeled steps are indicative of one embodiment of the presented method. Other steps and methods may be conceived that are equivalent in function, logic, or effect to one or more steps, or portions thereof, of the illustrated method. Additionally, the format and symbols employed are provided to explain the logical steps of the method and are understood not to limit the scope of the method. Although various arrow types and line types may be employed in the flow chart diagrams, they are understood not to limit the scope of the corresponding method. Indeed, some arrows or other connectors may be used to indicate only the logical flow of the method. For instance, an arrow may indicate a waiting or monitoring period of unspecified duration between enumerated steps of the depicted method. Additionally, the order in which a particular method occurs may or may not strictly adhere to the order of the corresponding steps shown. 
         [0066]      FIG. 6  illustrates one embodiment of a method  600  for detecting the formation of a short between a MR head  406 - 408  and a head substrate  402 . In one embodiment, the method starts  602  with generating  604  a baseline electric potential between a head substrate  402  and ground using the biasing module  202 . The monitoring module  204  may then monitor  606  the level of the electric potential generated  604  by the biasing module  202 . The detection module  206  may then detect  608  the formation of a short circuit between an MR head  406 - 408  and the head substrate  402  and the method  600  ends. In one embodiment, the detection module  206  detects the formation of a short by comparing the substrate potential level monitored  606  by the monitoring module  204  to a predetermined threshold value. 
         [0067]      FIG. 7  illustrates a further embodiment of a method  700  for detecting the formation of a short between a MR head  406 - 408  and a head substrate  402 . In one embodiment, the method  700  starts  702  when the biasing module  202  generates  704  an electric potential of 1.5 Volts between a head substrate  502  and ground  510 . The monitoring module  204  may then monitor  706  the level of the electric potential. When a magnetic tape cassette  108  is ejected  708  from the magnetic tape drive  106 , the detection module  206  may check  710  the level of the electric potential. 
         [0068]    If the level of the electric potential has dropped  712  to about 0.9 Volts, the detection module  206  may detect  714  the formation of a short circuit  418  between one of the MR heads  406 - 408  and the head substrate  402 . The notification module  308  may then notify  716  a user of the presence of a short circuit  418 . If the potential level did not drop to around 0.9 Volts, the monitoring module  204  may continue to monitor  706  the level of the potential difference. 
         [0069]    In one embodiment, a user may insert a head cleaning device  110  and clean  718  the MR heads  406 - 408  in response to a displayed error message. Alternatively, the cleaning module  306  may manage an automated cleaning  718  of the MR heads  406 - 408 . When the MR heads  406 - 408  have been cleaned  718 , the monitoring module  204  continues to monitor  706  the potential level, and a new magnetic tape cassette  108  may be inserted into the magnetic tape drive  106 . 
         [0070]    The disclosed embodiments of the apparatus  200 ,  300 , system  100 , and method  600 ,  700  for detecting formation of a short between an MR head  406 - 408  and a head substrate  402  provide the necessary benefits of early detection and correction of shorts between an MR head  406 - 408  and the head substrate  402 . Consequently, time and system resources are saved while providing greater data reliability. 
         [0071]    The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.