R. A. TRACY
T
HE PRESENT STATUS of the co-incident-current matrix memory is largely one of hope for the future, in so far as very large memory systems are concerned. Memories made to use pres-ently available materials are limited by cost and associated engineering problems.The problems arise largely from the use of ferrite cores. The major problem is the nonuniformity of the cores. Pro-curement, core testing, matrix wiring, matrix testing, and core replacement also present serious difficulties. Complicating these problems are the material proper-ties of the ferrites, such as temperature and strain sensitivity, the necessity for operating on a minor hysteresis loop, and the difficulties encountered in varying the magnetic parameters to fit a core to a system.
Description of New Memory Element
A new magnetic memory element has been discovered by Douglas Wendell of Burroughs Corporation. It consists of a short length of 4-79 Molybdenum Perm-alloy ultra-thin tape wrapped directly around a bundle of insulated wires and then glued. Although the material has a relatively large coercive force, the small diameter obtained by wrapping directly on the wires more than compensates for this, so that the current required to switch the c;ore is comparable to that required by the ferrites. The tape is used as it is received from the rolling mill. No heat treatment is required. The original con-cept specified that this tape, wrapped on four wires, constituted a memory element.
Fig. '1 shows such an element, and sug-gests how the original name of "bug"
evolved. The individual elements were then soldered into a matrix array. The first matrix made in this way is shown in Fig. 2.
While this arrangement for a complete matrix array has proved to be very ad-vantageous, the fact that there are a number of solder connections for each element introduces a rather extensive fabrication operation. Examination of the wiring array was made to determine possibilities of reducing the number of cOllllections. It was found that a string
R. A. TRACY is with the Burroughs Corporation Research Center, Paoli, Pa.
of elements having three common wires and one unique wire was possible. The matrix could then be made in halves as shown in Fig. 3. A machine was designed and built to perform the wrapping opera-tion on a semi-automatic basis.
The problem of the numerous solder connections was, of course, referred back to the original inventor and his associates.
It was then pointed out that the tape could be wrapped at the appropriate stations in a pre-wired matrix array. A matrix made using this new technique is shown in Fig.
4. Several of these matrices have been built to test uniformity and to study tech-niques. Refined techniques of wrapping are available whereby many cores can be wrapped simultaneously.
Properties of New Memory Element
The properties of the metal tape elim-inate many of the problems associated with the ferrite cores. The 4-79 Molyb-denum permalloy ultra-thin tape has a 60-cycle per second hysteresis character-istic as shown in Fig. 5(A). Minor loops are included to provide a better com-parison with the ferrite material which is shown in Fig. 5(B). These figures in-dicate the reason for outer-loop operation with the metal tape in contrast to the
inner-loop operation required with the ferrite. Coincident-current operation would not be possible using the outer loop of the ferrite. The value of the field at the knee of the hysteresis loop does not vary greatly in the different loops for the metal. This is not true in the case of the ferrite. Thus, the inner loop opera-tion makes the configuraopera-tion sensitive to transient currents which can drive the core to an outer loop, and in this way re-quire demagnetization before using.
Other material properties of interest are temperature, sensitivity, strain sensi-tivity, and uniformity of magnetic proper-ties. The temperature sensitivity is shown in the graph of Fig. 6. The lower temperature sensitivity of the metal tape is understood when the Curie temperature is considered. The ferrite has a Curie temperature under 200 degrees centigrade which is less than half the metal Curie temperature of 430 degrees centigrade.
The closer the operating temperature is to the Curie temperature, the greater will be the temperature sensitivity.
Sensitivity to strain is negligible in the metal tape. The material has been strained to the breaking point with no noticeable change in properties. This is understandable when it is recalled that a 95% cold reduction occurred in the rolling process, and no strain relief anneal has been applied. The cores may be potted directly in an epoxy resin with no change in properties. The uniformity of the material is very good for tape from the same melt with the same processing history. Deviations from uniformity Fig. 1 (left). Bug memory element Fig. 2 (below left).
First matrix memory Fig. 3 (below). Sec-ond matrix memory
Tracy-Megabit Memory
Fig. 4. Final matrix memory
within a batch are due to mechanical tolerances in thickness. This is a maxi-mum of 10% and effects only the total flux. Large deviations exist for material from different melts, and smaller devia-tions for material subjected to different processing schedules. This, however, is not serious because a 50-pound melt will make between 10 and 20 million memory elements.
The behavior of the memory element under pulsed operation is quite similar to the ferrites. Fig. 7 shows the output voltage of a ferrite and the metal under identical operating conditions. A graph of th~ variation of output signal and noise versus driving current provides a more detailed comparison of these elements.
This is shown in Fig. 8. It can be seen that the metal tape is usable over a wider current range.
A very important advance is the vari-ability of pulse operation that can be ob-tained by varying dimensional parameters.
A longer metal tape or a wider tape will give a greater output voltage. A varia-tion in wire size varies the core diameter allowing the driving currents to be pre-selected. The range of half-select cur-rent was from 150 milliamperes to an unlimited amplitude for the material as shown in Figs. 5 and 7.
The full range of properties attainable for various materials and processing tech-niques is not known. The majority of work has been done on metals which can replace the slow (5 second) ferrites.
Materials which can operate at much Tracy-Megabit Memory
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higher speeds have been observed. The latter, of course, requires higher driving currents.
Cost Considerations
An analysis of the cost of these memory elements is in part a guess since they are not yet in production. Materials cost per element is 3 cents. Testing before fabrica-tion of 'the matrix is not required once a batch of material has been selected. Fab-rication cost includes pre-wiring the matrix and wrapping the cores. This can be
accomplished by automatic means, this should not exceed 2 cerits, and will prob-ably be less. Testing of the matrix is identical to ferrite planes. However, re-placement of cores which are not within specifications is simply a matter of remov-ing the old tape and rewrappremov-ing a new one at that station. No wiring is involved.
This provides a factor of ten cost ad-vantage over ferrite cores and also a time advantage in fabrication. Further ad-vantages can be realized in associated memory equipment such as drivers and sense amplifiers. The superior
proper-ties of the metal tape permit the use of less critical specifications on these equip-ments, thereby permitting an additional cost reduction.
Conclusions
This new memory element removes many of the limitations presently restrict-ing matrix memory size. Uniformity, cost of materials, cost of fabrication, and testing are all improved to the extent that million-bit matrix memories be-come quite feasible.
---+---Discussion
G. Myers (U. S. Air Force): Is there any trouble getting uniform core diameters?
How are diameters controlled?
Mr. Tracy: The information on how the magnetic tape is wrapped around the wires cannot be released right now. Uniform diameters are gotten, though. One way of determining this is to measure the value of the current required to begin to upset the flux of the core. This measures the value of the knee of the hysteresis loop, which in turn measures the core diameter since the loops of the material are uniform.
D. Meier (National Cash Register Corpora-tion): Are the switching times comparable to those of conventional S-3 ferrite cores?
Mr. Tracy: It was attempted to replace directly the S-3 core. When the diameter which could best be used with the currents required for the S-3 was obtained, the switching time was 4 rather than 5 micro-seconds.
R. Schultz (General Electric Company) : What was the thickness of the tape?
Mr. Tracy: 1/8 mil.
K. Preston, Jr. (Bell Telephone Labora-tories) : How large a working memory has been constructed from these elements?
What tolerances have you been able to maintain on the magnitude of half-select current required for a batch of elements?
Mr. Tracy: Twenty by 20 planes have been fabricated and tested. A working memory was not constructed. Secondly, the con-dition was imposed that the drive current must be able to vary by 10% and the cores must fit the system. The system then was not analyzed to determine if the spread was below 10%. The 3-sigma value was within that 10% tolerance.
W. W. Davis (Naval Ordnance Labora-tories) : How do you get even reasonable loop squareness following the wrapping operation?
Mr. Tracy: The material, as received from the rolling mill, has a high degree of strain anisotropy. This affects the magnetic properties, producing an anisotropic mag-netic material such as 50-50 nickel-iron.
Since the material is insensitive to the strains encountered in the wrapping opera-tion, the squareness was retained.
R. F. Mauger (National Cash Register Cor-poration): Is core loss due to eddy currents a limiting factor at very high frequencies?
Mr. Tracy: In an operating matrix mem-ory, the frequencies encountered are below those that will give eddy-current losses in 1/8-mil tape.
D. R. Brown (Lincoln Laboratories, Massa-chusetts Institute of Technology): Are the curves in Fig. 8 for a memory array, a plane, or a single core? How is signal-noise defined?
Mr. Tracy: The curves in Fig. 8 are for a single core. Signal-noise is defined by
measuring the noise and signal at the peaks.
W. J. Bartik (Sperry Rand Corporation):
What output voltages are obtained for a one? How many wraps of 4-79 Molyb-denum permalloy are used?
Mr. Tracy: This depends on the length of tape that is used. If a 1/2 inch length of tape is used, the output will be approxi-mately 30 to 40 millivolts. If 1 inch of tape is used, the output will be approximately twice as much. The 30 millivolt output was aimed for. This can be varied by varying the length of tape. The number of wraps are not controlled, the length of tape is controlled; a 1/2 inch was the standard length.
R. M. Clinehens (National Cash Register Company): I would like to ask where the unannealed 4-79' Molybdenum permalloy can be purchased and what is the over-all dimension of a single, finished core? I know that you have not yet fabricated an array of this type as large as a million bits, yet I would like to ask what your estimate would be of the dimensional volume space that would be occupied by such an array?
Mr. Tracy: The unannealed 4-79 Molyb-denum permalloy tape can be purchased from Armco. The dimensions of a single core are 1/8 inch long by approximately 20 mils diameter. The miniaturization will probably be limited by the number of con-tacts that must be made to a plane. The memory plane can be folded several times after fabrication to realize this small space.