CA1038077A - Mutually exclusive magnetic bubble propagation circuits - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Closed loop propagation channels for magnetic bubbles are mutually exclusively accessed by means of two different sets of repeating sequences of discrete, pulsed magnetic drive field orientations. A closed loop bubble propagation circuit is driven by a repeating sequence of pulsed orientations in which a plurality of interlaced subsets of the sequence operate corres-ponding sections of the circuit. A closed loop zigzag circuit traversing sections of the loop parallel to the sides of an equilateral triangle is driven by a sequence of pulsed field orientations aligned respectively with the sides of the equilateral triangle. Continuous overlay circuits are introduced driven by sequences of discrete nonorthogonal, pulsed drive field orienta-tions repetitively realigning with consecutive segments of the overlay circuit.

Description

~03~07~7 The invention relates generally to the field of mag-netic bubble technology (MBT) and, more particularly, to means for propagating or transmitting magnetic bubbles, especially in recirculating closed loops~
This application is a divisional of copending Canadian Application Serial Number 212,765, filed October 31, 1974.
Briefly, MBT involves the creation and propagation of magnetic bubbles in specially prepared magnetic materials. The word "bubb1e", used throughout this text, is intended to encom-pass an~ single-walled magnetic domain, defined as a domain having an outer boundary whichcloses on .itself. The applica-tion of a statlc, uni~orm magnetic bias field orthogonal to a sheet of magnetic materiAl havilly ~u:itable un:laxtal antsotropy causes the normally random serpentine pattern of magnetic domains to shrink into short cylindrical conigurations or bubbles whose co~non polarity is opposite that o~ the bias field. The bubbles repel each other and can be moved or propagated by a magnetic field in the plane of the sheet.
Many schemes exist for propagating bubbles along pre-determined channels. These techniques can be classed generally as conductor-accessed and field-accessed. In conductor-acces-sed propagation systems, loops of electrical conductors are dis-posed in series over the magnetic sheet. A propagation system has been proposed using a "serpentine" conductor criss-crossing a erromagnetic rail defining stable complementary bubble posi-tions. In field-accessed propagation systems electrical con-ductors are not disposed on the magnetic sheet for propagation;
inst.ead, an overlay pattern of ferromagnetic elements estab~
lishes a bubble propagation channel in which a sequence of ., ~

---~ C-07-21-0237 .. ' .

~(~3~7 attracting poles is caused to be formed in the presence of a continuous, uniformly rotating magnetic drive field in the plane of the sheet. A major distinction in function between conductor-accessed and field-accessed circuits is that several conductor--accessed circuits can be disposed on the same sheet or 'lbubble chip" and operated completely separately and ex-clusively from each other, while field-accessed circuits on the same chip ~1 operate at the same time under the control of an ubiquitous, uniformly rotating, common drive field.
One attempt at providing field-accessed circuit selection in the prior art is shown in U.S. Patent No. 3,543,25~ to Perneski illustrating several variations on the famillar T~bar circuit driven by diffexent perrnutations of pulsed orthogonal drive fields.
MBT can be used in data processing beca~se magnetic bubbles can be propagated through channels, whether field accessed or conauctor-accessed, at a precisely determined rate so that uni-form data streams of bubbles are possible in which the presence or absence of a bubble at a particular position within the . stream indicates a binary "1" or "0". Because of its potential for low cost, low power consumption and extremely high bit density, MBT is under active consideration for use in large scale relatively low speed memories. One of the prime design elements of many memory systems utilizing field-accessed magnetic bubbles is the provision of a seriai closed loop bubble path which can be used as a recirculating "shift registerl'. Many memory arrangements of this type employ a plurality of "minor"
loops selectively interconnectibl~ with a "major" loop such that bubbles can be transferred between the major and minor loops on command. The ability to propagate bubbles in ~Q3~

one recirculating loop without operatin~ other loops on the same chips has in the past been con~ined to systems employing conduc-tor-accessed circuitsO
Another consideration to~hich this application is dir-ected is the desirability of providing a segmented continuous overlay rather than discrete spaced e].ements such as those used in chevron and T-bar circuits. U. S. Patent No. 3,518,643 to Perneski illustrates zigzag and crenellated forms of continuous overlay patterns based on right angles, driven by a pulsed ortho-gonal coil arrangement.
In a preEerred embodiment o~ the p~es~nt .~nvention there is prov.~ded a closed loop magnet.ic bubb.le propa~tion clr-cuit comprising a sheet Oe magnetic hubble material, means ~or applying a magnetic bias ield orthogonal to said sheet to pro-duce and maintain magnetic bubbles therein, a continuous ferro-magnetic overlay circuit operatively disposed on said sheet forming a closed bubble propagation path, and ~ans for applying a pre-determined sequence of pulsed discrete drive field orienta-tions in the plane of said sheet for propagating bubbles around said closed path.
In a further embodiment of the present invention there is provided a closed loop continuous magnetic bubble propagation circuit, comprising a sheet of magnetic bubble material, means for applying a bias field orthogonal to said sheet to produce and maintain bubbles therein, and a continuous magnetic bubble propagation overlay circuit on said sheet in the orm of a con-tinuous zigæag pattern crisscrossing the sides of a reference triangle such that the zlgzag elements o said pattern on any given side of said crisscrossed triangle are parallel to the other two sides o said reference triangle, and means for applying a predetermined sequence of pulsed magnetic drive fields in the ~ ~ 4 ~

plane of said sheet in fi~st/ second and third directions res-pectively parallel to the sides of said reference triangle for propagating bubbles around said closed path.
In a still furthe~ embodiment of the present invention there is provided field-accessed mutuall~ exclusive closed bubble propagation paths, comprising means defin.ing a first many-sided closed bubble propagation path in which the sides of said path are parallel to corresponding sides of a first equilateral ref-erence triangle.having a first orientation, means for applying a se~uence of three pulsed drive fields aligned respectively with the sides o said first equilateral tr.iangle ~or propagati.ng bubbles around sa.id :Eirst alosed path r each s.ide of said .E.irst closed path being ~ormed by a circu.it pa~tern overlay for propa-gating bubbles along said side in response to alternating align-ment of said first se~uence of drive fields with the other two sides o said first reference triangle, a second many-sided closed bubble propagation path the sides of which are parallel to corresponding sides of a second equilateral reference triangle having a second orientation such that the sides of said second triangle are nonparallel to the sides of said first triangle, means for applying a second sequenceof three pulsed drive fields aligned respectively with the sides of said second triangle, each side of said second closed path being formed by a circuit pattern overlay for propagating bubbles along said s.ide in response to alternating alignment of said second sequence of pulsed drive fields with the other two sides of said second triangle.
One of the objects of the invention is to provide closed loop bubble propagation channels, field-accessed by means of nonrotating, i.e. discretely orientedl pulsed fields. An-other object of the invention is to provide mutually exclusive, closed loop circuits, field-accessed by means ofdifferent sets ~ - 4 a -q7 of pulsed field orientations. A further object of the invention is to improve continuous overla~ ~ield~accessed circuits, which ensure uniform bubble size by maintaining uniform bias field strength due to the absence of discrete, spaced circuit elements.
The applicants have discovered that these and other objects of the invention are accomplished by providing a closed loop _ 4 b -~Q3~0~77 bubble propagation circuit driven by a pulsed drive field assuming discrete orientations in a predetermined repeating se~uence. In the preferred embodiment, subsets of a sequence of nonorthogonal field orientations are effective to drive bubbles on corresponding sides o~ the closed loop path. A
continuous zigzag form overlay circuit traverses sides of the closed path parallel to the sides of an equilateral triangle.
The drive field sequence includes three discrete orientations corresponding to the sides of the triangle. Any two of the orientations are effective to drive bubbles along one side of the closed path. More complex forms of closed loops based on ; the equilateral triangle can be generated by usin~ any closed loop zigzag circuit design as a constructional l:in~ on which to build successive zigzag circuits in the nature of a "Peano diagram".
Pulsed field-accessed, closed loop circuits are applied, in another important aspect of the invention, to the problem of field-accessed circui~ selection. A pair of closed loop pulsed field-driven circuits are oriented relative to each other such that a sequence of pulsed field orientations effec-tive to circulate bubbles on one of the closed loops is inef-fective to operate the other closed loop, and vice versa.
Hence, the closed loops, while field-accessed, are mutually exclusive.
A semihexagonal continuous overlay circuit is driven by discrete pulsed field orientations repetitively realigning with consecutive angled segments of the overlay circuit.

-" C-07-21 0~ ~

3~7 BRIEF DESCRIPTION OF THE DR~WINGS

Fig. 1 is a fragmentary perspective view of a bubble chip furnished with a conventional chevron circuit.
Fig. 2 is a schematic drawing illustrating a semihexagonal continuous overlay and the associated drive field sequence according to the invention.
Fig. 3 is a schematic drawing illustrating a zigzag con-tinuous overlay circuit and the associated drive field sequence according to the invention.
Fi~. 4 is a schematic diagram illustrating a closed loop .
zigzag continuous overlay circuit and its associated drive field acaording to the invention.
Fig. S is a schematic diagram illustrating a variation on the closed loop zigzag circuit of Fig. 4.
Fig. 6 is a schematic diagram indicating a technique for generating successively more complex closed loop zigzag cir-cuits.
Fig. 7 is a schematic diagram illustrating a pair of mutually exclusive field-accessed closed loop circuits and their respective drive field sequences according to the invention.
~ ig. ~ is a graph indicating the hysteresis curve for a particular overlay material.

DESCRIPTION OF P:REFERRED E~ DIMENTS

Fig. 1 illustrates the basic components of a field-accessed garnet bubble chip haviny a conventional chevran circuit~ A
substrate 10 of nonmagnetic garnet supports an epitaxial magnetic bubble garnet layer 12 and spacing la~er 14 of silican C 07-2~ ?37 ~38~
oxide to which conventional permalloy chevron circuit elements 16, 18 and 20 are bonded. The chip is subjected to a static magnetic bias ~ield orthogonal to the plane of the magnetic bubble garnet layer 12. In the presence of a bias field of suitable strength, cylindrical magnetic bubbles (not shown in Fig. 1) are mai~tained in the bubble garnet layer 12. Conven-tionally, a rotating, in-plane magnetic drive field, produced by an orthogonal pair of Helmholtz coils, causes bubbles to pro- I~
pagate from chevron circuit element 16 to element 18, for example.
~,~ Many parameters affect the performance of che~ron circuits such as the number of parallel chevrons per bubble position (single chevrons are illustrated in Fig~ 1), thc spacing of adjacent chevron elements, their width, the magnetic properti~es of the overlay material, and the strengths of the magnetic bias and drive ~ields.
One of the problems with discontinuous circuit overlays of spaced elements, like the chevron circuit ~Fig. 1), or the familiar T-bar circuit is that bubbles propagating along the cir-; cuit channel are subjected to varying bias field strength as they move from under one circuit element through a gap region to the next circuit element. A continuous circuit overlay avoiaing this problem is shown in Fig. 2. The circuit overlay 22 is composed o~ a series of parallel semihexagonal, three-segment sections, each comprising an angled straight line segment 24 coupled by a horizontal straight segment 26 to another angled straight segment 28, to form a truncated sawtooth pattern. Segments 24 and 2~ ¦
make an angle of 120 with respect to the horizontal segment 26 and the angled segments make an angle ~-60 with respect to each r~ C-07-2~-0~37 ~ 0380~77 other. Thus the segments 24, 26 and 28 together outline one half o~ a hexagon. The semihexagonal sections are continuously interconnected by joining adjacent angled end segments 24 and 28.
A pulsed field sequence 30 for propagating bubbles along the continuous overlay 22 in Fig. 2 comprises three discrete pulced field orientations parallel to the elements 24, 26 and 28, respectively. The field sequence 30 occurs in the order indicated by the numerals associated with the field vectors.
The first field orientation is aligned with segment 24, the second orientation is aligned with segment 26 and the third is aligned with segment ~28. The three orientations all point in the same general direction of travel along consecutive seg-ments in the circuit 22~ The alignment and diraction o the pulsed fields orms attrac~ing poles along the line segments of the semihexagonal overlay in the sequence indicated by the numerals 1, 2 and 3 adjacent to the circuit 22. Bubbles ~not shown) under the circuit overlay 22 are attracted to the pole positions in the order indicated and thus traverse first the segment 24 to the position 1, next the segment 26 to the position

2 and finally the segment 28 to the position 3. This motion is repeated for each repetition of the fîeld sequence 30 as bubbles move over successive semihexagonal sections in the circuit 22. ' Fig. 3 illustrates a zigzag continuous overlay channel 32 comprising alternately angled straight line segments 34 and 36 forming a regular sawtooth pattern. ~he segments 34 are parallel, as are the segments 36 which meet the segments 34 preferably at a 60 anglej 0. The pulsed drive field sequence 38 comprises a pair of discrete, alternate orientations labeled 1 and 2 separated by the angle 180 minus ~ or preferably 120.

~ C-07-21-0237 . ' ~L~380~7 These orientations are parallel respectively with segments 34 and 36. Bubbles (not shown) under the overlay channel 32, attracted to'the changing poles formed by the drive field 38, are propagated to the right, the direction of the drive field 38 as viewed in Fig. 3, through the bubble positions labeled ,1 and 2 form~d at the upper and lower vertices of the circuit respectively.
Fig. 4 illustrates the manner in which, closed loop con-tinuous zigz-ag type circuits are constructea. The equilateral tr;angle 40 in Fig. ~ is drawn for reerence as a construc-tion,al diagram. The overlay c.ircu.it itse.l.~ is represented by the closed xiyzag patt~'rn ~2 consccu~iv~ly cri.s.scrossing khe sides of the triangle 40. The z.igzag cixcuit 4Z, itself has three sides or sections 4~, 46 and 48 corresponding to the sides a, b and c of the equilateral triangle 40. For each section 44, 46 or 48 the straight line segments of the zigzag pattern traverse or crisscross one side of the triangle 40, and ' the segments are alternately parallel to the other two sides of ; the-same triangle. Thus section 44 of the zigzag circuit 42 .
crisscrosses side a of the triangle and every other parallel indiviaual element of the ~igzag section 44 is parallel to side b or side c. Th,e section 46, on the other hand, criss-crosses side b and is alternately parallel 'to sides a and c.
Similarlyt,section 48 cr;sscrosses s.ide c and is alternately parallel to sides a and b.
Pulsed drive field 50, shown in Fig. ~ to the right of the closed zigzag circuit 42, comprises a sequence o~ three pulsed field orientations, labeled arbitrarily 1, 2 and 3, which are aligned with sides b, a and c respectively of the ` C-07 21-n237 construction triangle 40. Recalling from Fig. 3 that alter-nately pulsed ~rive fields parallel to the individual straight line segments composing a zigzag overlay channel and pointing in the same direction down the channel will drive bubbles in that direction along the channel, it can be seen in Fig. 4 that for any given section, 44, 46 or 48 only two of the three pulsed field orientations successfully propagate bubbles in tha~ section. For example, in section 44 the drive field orientation labeled 2, parallel to side a of triangle 40, has little effect on the propagation of bubbles in section 42.
However, ield orientations 1 and 3 are effective to drive bubbles to the right (clockwise) along section ~4 in the man-ner shown .in Fig. 3. ~n sec~ion 46, sirnil~rly, the ~field orientation 1 parallel to the traversed side b is ineffective to propagate bubbles while the other field orientations 2 and

3 are effective to propagate bubbles along this section.
Section 48 operates in the same manner driven by field orienta-tions 1 and 2. Hence, the effective field orientations for each-side or section of the closed loop circuit 40-are inter--lacea in the drive sequence. The fact that bubbles in one sec-tion of the circuit 42 are not in motion during the application of one of the three pylsed drive field orientations has no adverse effect upon the running of the circuit.
The equilateral triangle 40 on which the circuit 42 is based is employed hecause it provides maximum isolation of the three field orientations. That is, this arrangement perm;ts the field orientation parallel to the traversed side to have mi~imum effect on a zigzag circuit section corresponding to .
~ 380~q that side. ~ile 60 is beli~ved to be the best angle for : achieving this type of circuit, clearly analogous circuits with angles di~fering from 60 are possible in achieving closed loop configurations. In adaition, constructional figures, like triangle 40 in Fig. 4, do not have to be triangles but may be any polygon having any number o~ sides restricted to three orientations, preferably at 60 angles, where circuit sections along any parallel sides will not be required to propagate in opposite direction~. .
Another example of the closed loop, continuous zigzag cir-cuit based on a 60 angte is sho~n in ~ig. 5 in which the ; construct.ional fiyure 52 consists o a hor.izontal line .int~r-connecting the ends of a sexies of thrcc sawtee~h. q'he ziyzag ~ircu;t S4 is drawn so as to crisscross the respective sides of the figure 52 to form a closed loop. The resulting circuit is driven by pulsed drive field 50 in Fig. 4 in a manner simi-lar to that for the regular triangular circuit 42.
. - Another closed loop circuit is shown in Fig. 6 dr.iven by the pulsed drive field sequence 50, where the circuit 42 of ~0 Fig. 4 itself forms a constructional figure 42 to generate a more complex form of.zigzag circuit 56. Again all of the elernents of the resulting zigzag circuit 56 are alternately parallel to respective ones of the sides o~ the original triangle 38 and propagation along parallel sections is in the same direb~ion. In a sirnilar manner the co~nplex æigzag circuit 56 can be used itself as a constructional figure to produce a third generation closed loop zigzag circuit still more complex.
This process of producing successive constructional figures for successive generations o~ zigzag circuits can be carried C-0~-21-0237 ~(~3!3i~
out as far as desixea, in the nature of Peano diagrams in the field of topology, in oraer to efficiently utilize the space available on the bubble chip. No matter how complex the closed zigzag pattern becomes, it will be composed of zigzag sections whose general direction o~ propagation is parallel to one of the sides of the original equilateral triangle, and thus propayation over this section will be affected by two of the three pulsed field orientations 48.
It is important to note that these closed loop circuits can not be driven by a uniformly, rotatlng magnet.ic field; they must be driven by a se~uence of pulsed fields such as .ield sequence 50.
Fig. 7.illus~rates the pri.nciple o~ mu~ually exclusive field-accessed, closed loop circu.its. In this embodiment, a pair o closed loop zigzag circuits 58 and 60 are based respectively on equilateral triangles A and B, symbolizing the two closed loop circuits. Because of the offset angulax ~orientations of the circuits A and B, when these Cil^cUits are placed on the same chip, one may be driven by one sequence of . pulsed dirve fielas while the other is inoperative and vice versa. In particular, circuit B is tipped approximately 30 relative to the orientation of circuit A. Because the cir-cuits A and B are both built on 60 angles, the three sides of circuit B each make an angle of 30 with the three corre-sponding sides of circuit A. Expressed differently, the sides of triangle B are orthogonal to the sides of triangle A.
This angular separation is sufficient to render the subset lA, 2A and 3A of the pulsed arive field sequences 62 inoperative to drive bubbles on the circ~it B while it suc~essfully c-07-2l-n2 37 1~3~07q .
propagates bubbles on the circuit A. Conversely, the subset lB, 2B and 3B, offset by 30 from the A subset, is ineffective to propagate bubbles in circuit A while successfully moving bubbles around the circuit B. Hence circuits A and B are mutually exclusive, field-accessed closed loop circuits~
The reasons why these particular circuits in Fig. 7 are mutually exclusive can be explained, for example, in terms of the action of the A sequence on the B circuit. Assume that the vectorial projections of the A orientations on corresponding segments of the B circuit are sufficient to affect the magnetic polarity of these segments. When the ~ield 3A ig applied to the righthand section ~illustxated) o~ circuit 60(B), the field 3A, as the bisector of the anyles rnade by the segments, affects adjoining segments equally. Thus a bubble 64 at an inner vertex, for example, will experience zero net attraction to move to an outer vertex. The next consecutive field orientations. lA and 2A, will leave the bubble 64 where it is because they (i.e., their strongest vectorial projections) point along the adjoining segments toward the location already occupied by the bubble 64.
2Q . This method of analysis holds true for any section of circuit B
under consideration, and extends by direct analogy to the effect of field sequence B on circuit A.
While the embodiments diagrammed in Fig. 7 indicate a so-called "first generation" zigzag circuit configuration or circuits 58 and 60 with sections corresponding directly to the sides of the original constructional triangle, it should be ; clear that more complex forms can be used so long as the individual line segments making up the closed circuit zigzag overlay are parallel to the sides of a triangle, `\
~ C-~7~21-~237 .
pre~erably equilateral, parallel sections propagate in the same direction and the triangles are sufficiently skewed relative to each other. As in the description of Figs. 4 through 6, 60 angles are preferred although operative embodi-ments are not limited strictly to 60 angles, triangular con-figur~tions other than equilateral being feasible. Similarly, the principle of mutual exclusivity in field-accessed, closed loop circuits illustrated in Fig. 7 is not necessarily limited to continuous overlay zigzag circuits. ~iscre~e circuit el~ment, closed loop overlay patterns mu~ually exclusively op~rable by means of pulsed field orientations represent a feasible exten-sion of the underlying principle of field-accessed closed loop mutual exclusivity.
. As illustrated in the pulsed drive field sequences 62 in Fig. 7 the angle between sequence A and sequence B is 60. Thus, for example, field 3A has a vectorial projection in the direction of field 3B whose magnitude is the cosgin of 30. Thus the field strengt~ in the direction 3B attributable to a pulsed field in the direction 3A would be 86% of the full magnitude in the field in the direction 3A.
To enhance the exclusive operation of circuits A and B, the circuit material used for the continuous zigzag circuit, or whatever type of circuit is employed, is chosen with the right hysteresis curve such that saturation occurs in the presence of full field strength when the field is strictly aligned ~)38~7 with the element in circuit ~ for example, while 86% field strength operating on t~e correspond~ng elements of circuit B
is insufficient to saturate these elements, that is, it is not enough to switch the magnetic polarity of the corresponding elements. One example of a suitable hyste~esis curve is shown ~n ~igure 8. A material whose magnetic properties approximate the h~steresis curve in Figure 8 is the material known under the trade name Perminvar.
The same general hysteresis property may also be ob-tained by depositing materials for the circuit overlay which have a tendency to remain magnetized perpendicular to their long axes. Such materials will evidence magnetization parallel to their long axes only above a certain critica:L eield strength~
~ccoxdingly, the magnetic behaviour of such materials would also approximate the hysteresis loop shown in Figure 8.
Yet another way of enhancing mutual exclusivity would be to drive the circuits A and B at the highest possible speed to take advantage of the restrictive operating margins of the c~rcu~ts. Thus the A sequence of pulses might be well within the operat~ng margins of-circu~t A. At-the same time, because o~ reduced components o~ sequence A projected on circuit B, in combinatl-on with the hi~h operating speed, circuit B would not ; be withi~n its operating margins and thus would fail to function properly. All of these efforts described above have in common the pxinciple of improving mutual exclusivity by enhancing the criticality o~ the drive ~ield strengthsuch that the fully aligned field will cause saturation of parallel circuit elements while a field misaligned by as much as 30 will have an aligned component or projection which is not sufficient to saturate parallel circuit elements. Any parameter which trims the opexat~ng margins may be useful in this regard.

~ 15 -C-07~ 0237 ~338~77 The invention may be embodied in other specific forms without departing from its spirit or central characteristics.
The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalence of the claims are therefore intended to be embraced therein.

Claims (17)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A closed loop magnetic bubble propagation circuit com-prising a sheet of magnetic bubble material, means for applying a magnetic bias field orthogonal to said sheet to produce and maintain magnetic bubbles therein, a continuous ferromagnetic overlay circuit operatively disposed on said sheet forming a closed bubble propagation path, and means for applying a pre-determined sequence of pulsed discrete drive field orientations in the plane of said sheet for propagating bubbles around said closed path.

2. The circuit of claim 1, wherein said closed path has a plurality of sides, said sequence of pulsed drive fields containing a plurality of different subsequences for propaga-ting bubbles along corresponding ones of said sides.

3. The circuit of claim 2, wherein said subsequences are interlaced.

4. The circuit of claim 2, wherein said overlay circuit is continuous.

5. The circuit of claim 2, wherein said overlay circuit is continuous at least over each of said sides.

6. The circuit of claim 2, wherein said overlay circuit includes a many-sided closed configuration of zigzag circuits, the straight segments of said zigzag circuits being parallel to the sides of a triangle, said circuits being arranged such that bubbles traverse parallel segments in the same direction.

7. The circuit of claim 6, wherein said triangle is an equilateral triangle.

8. The circuit of claim 6, wherein each side of said closed path is formed by a zigzag circuit composed of alter-nately oriented linear segments parallel respectively with two sides of said triangle.

9. The circuit of claim 2, wherein said closed path includes a many-sided closed path in which each side is parallel to one side of a reference triangle, each side of said closed path being formed by a pattern of circuit elements for propa-gating bubbles in response to alternating alignment of said pulsed drive field with the other two sides of said triangle.

10. The circuit of claim 9, wherein said pattern is in the form of linear segments arranged end-to-end and alternately parallel to the other two sides of said triangle.

11. The circuit of claim 10, wherein said reference tri-angle is equilateral and said drive field sequence includes a repeating sequence of three pulsed fields consecutively angu-larly separated by 120°.

12. A closed loop continuous magnetic bubble propagation circuit, comprising a sheet of magnetic bubble material, means for applying a bias field orthogonal to said sheet to produce and maintain bubbles therein, and a continuous magnetic bubble propagation overlay circuit on said sheet in the form of a con-tinuous zigzag pattern crisscrossing the sides of a reference triangle such that the zigzag elements of said pattern on any given side of said crisscrossed triangle are parallel to the other two sides of said reference triangle, and means for app-lying a predetermined sequence of pulsed magnetic drive fields in the plane of said sheet in first, second and third directions respectively parallel to the sides of said reference triangle for propagating bubbles around said closed path.

13. Field-accessed mutually exclusive closed bubble propa-gation paths, comprising means defining a first many-sided closed bubble propagation path in which the sides of said path are parallel to corresponding sides of a first equilateral reference triangle having a first orientation, means for apply-ing a sequence of three pulsed drive fields aligned respective-ly with the sides of said first equilateral triangle for propa-gating bubbles around said first closed path, each side of said first closed path being formed by a circuit pattern overlay for propagating bubbles along said side in response to alternating alignment of said first sequence of drive fields with the other two sides of said first reference triangle, a second many-sided closed bubble propagation path the sides of which are parallel to corresponding sides of a second equilateral reference tri-angle having a second orientation such that the sides of said second triangle are nonparallel to the sides of said first tri-angle, means for applying a second sequence of three pulsed drive fields aligned respectively with the sides of said second triangle, each side of said second closed path being formed by a circuit pattern overlay for propagating bubbles along said side in response to alternating alignment of said second sequ-ence of pulsed drive fields with the other two sides of said second triangle.

14. The circuit of claim 13, wherein the sides of said second triangle are approximately perpendicular to the sides of said first triangle.

15. The circuit of claim 13, wherein said circuit pattern overlay for each side is composed of linear segments disposed end-to-end and alternately aligned with said other two sides of said first or second reference triangle.

16. The circuit of claim 15, wherein the magnetic proper-ties of said circuit elements are predetermined such that orientations of said first field sequence cause saturation of correspondingly aligned circuit elements in said first closed path but not of those in said second closed path and said second sequence of pulsed drive fields causes saturation of corres-pondingly aligned elements in said second closed path but not of those in said first closed path.

17. The circuit of claim 16, further comprising means for enhancing the criticality of the field strengths of said first and second sequences of pulsed drive fields to ensure mutual exclusivity of propagation in said first and second closed paths.