Magnetic bubble memory purge and verification system

#This invention relates to magnetic bubble memories and, more particularly, to a system for purging the contents of such a memory and providing a visual verfication of the purge.

#In recent years, magnetic bubble memories have gained in popularity over other non-volatile storage devices for secure data. This is because, unlike magnetic tapes or discs, no fringe field areas exist in magnetic bubble memories that can be analyzed for previously stored data. In a magnetic bubble memory, a single write pass of all data absolutely removes all previous data.

#When using a magnetic bubble memory for secure data, it is sometimes desirable to be able to completely erase, or purge, all of the data from the memory. If the built-in writing circuitry were utilized, the problem exists that the system supporting the bubble memory must be running and continue to run for the entire purge period. With a one megabit bubble memory, approximately ten seconds are required to completely rewrite the memory with bogus data. In an emergency situation, this time may not be available.

#The foregoing will be more readily apparent upon reading the following description in conjunction with the drawings in which like elements in different figures thereof have the same reference character applied thereto and wherein:

#FIG. 1 is an electrical schematic diagram of an illustrative system constructed in accordance with the principles of this invention; and

#FIG. 2 is a perspective view of a portion of an embodiment of this invention.

#A magnetic bubble memory typically comprises a magnetic domain film sandwiched between permanent magnets which provide a magnetic bias field to maintain the magnetic domains. This structure may then be surrounded by a shield. Since the magnetic properties of the magnetic domain film are temperature dependent, there is often provided a thermal compensation coil surrounding the shield, which coil is utilized to provide a magnetic field which modulates the permanent magnet bias field in accordance with temperature variations. If the plane of the magnetic domain film is considered the X-Y plane, then the thermal compensation coil may be considered to be along the Z-axis.

#If the current in the Z-axis coil is increased beyond the current required for thermal compensation, at some point the magnetic field generated by that coil will be sufficient to cause the magnetic bubbles to completely disappear. This will be substantially instantaneous, on the order of microseconds. It does require a complex operation to reseed and reboot the bubbles, but purging the memory is an emergency procedure making the rework costs worth it. No physical damage will occur and the rework procedure can be performed electronically on automated equipment.

#Referring now to FIG. 1, shown therein is illustrative circuitry for instantaneously erasing, or purging, the contents of the bubble memory, designated generally by the reference numeral 10. As part of the memory 10, there may be provided multiple Z-axis thermal compensation windings 12 connected in series. Each of the windings 12 is associated with one bubble memory device within the bubble memory 10. The diodes 14 and 16 and the transistor 18 form a portion of the thermal compensation circuitry, but do not form a portion of the present invention and are not utilized for purging the memory. According to this invention, connections are made at the points 20 and 22 to the compensation coils 12. These connections are brought out of the memory 10 via the leads 24 and 26.