1965,No. 3 67

Developments in the field of electronic computersduring the last decadeW. Nijenhuis and H. van de Weg

681.14-523.8

The rapid development of electronic computers as regards computing speed, reliabilityand capacity is mainly due to the advances made in the technology of transistors and diodes,magnetic memory cores and magnetic recording. These various points are discussed in thisarticle, and the possible lines along which present-day techniques may develop are indicated.

Introduetion

The electronic computer plays an indispensable partin modern society; we could not imagine being withoutit. If we realize that the first electronic computer,ENIAC, was completed in 1947 and that RemingtonRand installed the first commercial machine in 1951,then it will be clear that the development of thesemachines since then has been explosive. This rapiddevelopment comes out very strongly on scanningthrough the latest edition of "A survey of domesticelectronic digital computing systems" from the middleof 1961; but it mayalso be seen that a great deal ofpioneer work and experimentation was done in thoseearly years: of the list of computers given in this pub-lication, 85 of the 170 types considered are representedby a single model only, while about 90% of the totalnumber of machines manufactured belong to only15 types. We may assume that in that initial periodmoney was spent too easily for an attractive idea, abrilliant brainwave, without thinking out the conse-quences with respect to a complete system beforehand,with the result that the pilot model often did not gointo series production.The limited number of successes makes it easier to

pick out a few technical lines of development whiledisregarding the many side-tracks, and to give a briefaccount of these lines, as is intended here. We shallalso largely omit specialized developments, particular-ly those in the military sector. These may be interesting,and may sometimes be the precursors of the tech-niques of tomorrow as a result of the sometimes exor-

Ir. W. Nijenhuis and Ir. H. van de Weg (deputy director) areresearch workers at Philips Research Laboratories, Eindhoven.- This artiele is chiefly based 011 a talk which Ir. Van de Weggave to the Dutch Computer Society ill April1964.

bitantly high demands which are made; but the pricefactor is so subordinated to other arguments in thesecases that they cannot be taken as a real reflection ofthe present-day techniques.After the first period of amazement that it was

possible to make an electronic computer was over, allattention was focussed on the problem of reliability:next to the price the reliability is the most importantcriterion for judging a particular construction. On theone hand the reliability ofthe components is important,and on the other hand the reliability ofthe connections,e.g. plug connections and soldered connections. TheENIAC may be cited as an example of how thingswere with the reliability in the early days. When thismachine was switched on, usually at least a few ofthe 18000 electronic valves failed. A problem wasoften worked out several times, the solution beingconsidered to be correct if it was the same twice.It proved possible to increase the reliability of the

components by improved quality control, but initiallythis progress hardly kept pace with the increasing com-plexity of the machines, which led to much greaterchances of something breaking down. Many will stillremember the time before 1957, when computersonly did 7 hours' useful work in each planned 10 hours,and the average time between two breakdowns was2 to 4 hours. It was only once in a blue moon that amachine would work for a whole day without trouble.The situation is now much better. In the first place,

in addition to the above-mentioned quality control,better methods of construction have been introduced.These will be discussed below in more detail. But apartfrom this, various ways have been worked out to dealwith an error once it occurs. "Diagnostic" programmeshave been developed which make the computer itselfindicate which part of it has failed. The computer

68 PHILlPS TECHNICAL REVIEW VOLUME26

has also been split into a number of functional unitsand groups of functional units, with correspondingalarm' devices to localize the site of the trouble asclosely as possible. As a result, one can now find insales and hire contracts for electronic computers aguarantee that the average useful machine time will bemore than say 90%. The actual performances areusually even better. With a modern electronic com-puter, it should be possible to repair any trouble in lessthan half an hour.As a special example in this connection, we may

mention the new electronic telephone exchange of theBell Laboratories in the United States [1]. Electronictelephone exchanges have much in common withelectronic computers. They also use memories, in orderto be able to store subscribers' numbers temporarilyand they are built up ofthe same types of unit as com-puters (mainly flip-flops and gate circuits). These unitsare nowadays mounted on phenolic or glass-epoxyboards, with printed wiring ("printed-wiring cards").The designers of the Bell system have aimed at makingtheir diagnostic programmes so refined that the troublecan be pinned down to the printed-wiring board inquestion, so that maintenance can be done by unskilledworkers. Itmust be conceded that in a certain sense aneven greater reliability must be demanded of a tele-phone exchange than of normal commercial computersbut we can expect similar maintenance methods forcomputers in the future.

Although the remarks made so far about the relia-bility have been on the whole optimistic, we must notoverlook a few less happy points. In the first place, it istrue that the reliability of the electronic circuits is nowvery reasonable, but this ismuch less so with the electro-mechanical equipment which we find at the peripheryof the computer. Most trouble is still caused by thisequipment, and this can only partially be prevented byfrequent maintenance. Secondly, no computer is yetcompletely reliable in the sense that the user can besure that it will work at a given time, e.g. Wednesdayfrom half past two to five o'clock. This guarantee can-not yet be given for a computer, but it is demanded ofthe above-mentioned telephone exchange.

The reliability can be made especially great by in-creásing the redundancy of the circuit. For example,each valve can be replaced by two valves in parallel.A resistance R can be realized (as is also donefor amplifiers in under-sea cables) by means of fourresistors R connected two by two in series and inparallel, so that if one of these should break down orshort-circuit there will still be a connection via resistors.One has another form of redundancy if the systemcontains more than one switching unit for a givenfunction. The system can determine during operation

whether a switching unit is workingproperly, and ifnotcan look for an intact switching unit; the result is thata defect need not cause the whole system to ceaseoperation. Computer techniques could gain a lotfrom telephony techniques in this respect. Very little re-dundancy can be seen in the circuits of present-daycomputers, even in those cases where it is desired tohave a computer in service 24-hours a day without in-terruption. In this case a whole computer is often keptin reserve.We will now go into some more detail about the

history of the various techniques introduced into thefield of electronic computers in the course of their de-velopment, which apart from a reduction in the pricehave mainly led to an improvement in reliability. Weshall also pay a great deal of attention to the big in-crease in speed in nearly all parts of an electronic com-puter. '

Components and logic circuits

Ten years ago, valves (mainly double trio des) werestill in general use in computers, while active researchwas being carried out on the use of the point-contacttransistor, invented in 1948. In Britain [2] and theUnited States [31, some computers were indeed madewith point-contact transistors. This transistor can actas a high-speed electronic switch, with switching timesof 0.1 - 10 microseconds. It was however rather fragileand difficult to make reproducibly, so that it was quick-ly replaced by the junction transistor. This was origi-nally made by the pulling method, until the simpleralloying procedure was discovered. The junctiontransistor, originally announced as a low-frequencyamplifying element, was made suitable for faster andfaster circuits by improved methods of fabrication.While the best alloyed transistors made possible switch-ing times of about I (l.s,the introduetion of the diffu-sion technique has led to transistors which allowswitching times of a few nanoseconds. Some detailsabout these various types of transistors and their speedsare given in Table J.

Table I. Switching time obtainable with various kinds of tran-sistors.

Type of transistor Year of Switching timeIntroduetion (nanoseconds)

Point-contacttransistor 1948 10000-100

Junction transistor:Pulled 1949 dittoAlloyed. 1954 dittoDiffused 1956 100-5Planar 1960 100-5Field-effect transistor 1962 1000-20

1965, No. 3 DEVELOPMENTS IN ELECTRONIC COMPUTERS

The technique of epitaxial crystal growth and theuse of masking techniques have contributed to theimprovement of the properties of the transistor, e.g.to the lowering of the knee voltage of the transistorcharacteristic. Morèover, about-1957 the change wasmade from germanium to silicon, which led to a re-duced leakage current and better stability. The "planartechnique" allows P-N junctions in silicon to be pro-tected against the influence of the atmosphere by aSi02 film during their production, thus allowing afurther improvement in reproducibility and life. Thistechnique also proved to offer further possibilitieswhich we shall discuss below (in connection withmicrominiaturization).These methods of fabrication also allowed the pro-

duction of the field-effect transistor - long proposedon paper.The development of diodes more or less followed that

of the transistor, after the. change-over in about 1950from the old selenium rectifiers to germanium point-contact diodes. If diodes are to retain their rectifyingeffect even at high frequencies (high switching speeds)then a low value of the internal capacitance, amongother things, is of great importance. Point contactdiodes satisfied this demand, but like point-contacttransistors they were not sufficiently robust. Sturdierdiodes could be made by the alloying technique. Veryfast diodes can be obtained by a method in which thealloying is brought about during the attachment of thelead - a gold wire in this case. In general, diodes canbe made faster by introducing certain impurities("killers") into the germanium or silicon, of whichgold is the most important example. Finally, the intro-duetion of the Schottky diode, made by applying athin film of metal to the silicon, opened the possibilityof increasing the speed of the diodes even further. InTable 11, the switching times of the various diodes usedare shown.This table also includes details of two types of diodes

which are not used for their rectifying effect, viz thetunnel diode and the Boff diode. A few remarks fol-low concerning these two types.The tunnel diode, discovered by Esaki in 1958 has

two different functions as a result of the form of its. characteristic, viz, as bistable element and as amplifier.It initially seemed very promising as a high-frequencylogic element, owing to its inherently very fast switchingmechanism, but the difficulty ofmaking it reproduciblyand the low voltage difference between the two stablestates gave so much trouble in large systems thatthe interest in this component quickly faded. At pres-ent the tunnel diode is mainly regarded as a usefulauxiliary element for increasing the speed of transistorcircuits, e.g. the circuit for transmitting the carry to the

69

Table IT. Switching times of various types of diodes.

Switching time(nanoseconds)

100-0.1~5000

50-550-5 *)

<0.2

5-0.10.2

Type of Diode.1

Year ofIntroduetion

Point-contact diode 1947Alloyed 1950"Gold-bonded diode" 1955Planar 1960Schottky diode 1963

Tunnel diode 1958Boff diode 1962

*) If small and with gold killer.

following digit position in an adder; further as amemory element for small hyper-high-speed memoriesand for very high-speed counting circuits, mainly forapplications in nuclear physics.The Boff diode (also called the "snap-off diode")

which acts as a delay element giving delays of the orderof a few tenths of a microsecond, is of recent date. It isbased on the fact that an initially conducting diodewhich is suddenly exposed to a voltage in the cut-offdirection still passes current for a short time in thisdirection, the time depending on the value of the ori-ginal forward current. The conduction in the cut-offdirection ends with a very steep trailÏng edge whichgives the very short switching time. The possibilitiesofthis diode, e.g. in combination with the tunnel diode,are being investigated at present [41.

We shall now give a brief discussion of the mostwidely used passive elements (resistors and capacitors),together with a consideration of the development ofthe circuits.. At the start of the '50's, normal radio componentswere still used in combination with valves, which gaverather big constructions: the average packing densitywas about 0.1 components per cmê, As the componentsbecame more reliable and stable, their design wasaltered to match the functional construction of thelogic units in which they were used. This allowed ,thepacking density to be increased to about 0.5 compo-nents per ern". The introduetion of the transistor madeit possible to reduce the dissipation to much smallervalues, as a result of which resistors for lower powers(0.05 watt) and thus with smaller dimensions could beused.

(1] No. 1 Electronic Switching System, Bell Syst. tech. J. 43,1831-2592,1964 (No. 5, parts 1 and 2).(2] E. H. Cooke-Yarborough et al., Proc. lEE 103 B, suppl. No. 3,p. 361, 1956. .[3] J. H. Felkcr, Proc. IRE 40, 1584, 1952.[4] B. E. Sear, Charge controlled nanosecond logic circuitry,Proc. IEEE 51, 1215-1227, 1963.

----------

70 PHILlPS TECHNICAL REVIEW

Use of these components, combined with printedwiring on a laminated or glass-epoxy board, led to theabove-mentioned printed-wiring cards, which allowedthe packing density to be increased by a further factorof 5, to 2.5 components per cm'', This also greatly sim-plified the fabrication: all components could now besoldered on to the board in a single operation by dip

VOLUME 26

improving the reliability. In the above-mentioneddevelopment, known as microminiaturization, the cir-cuits are made by an integrated procedure, i.e. the con-nections between a number of components are madeat the same time as the components themselves. Onecan then expect higher reliability because less solderedconnections are needed in the circuits. One also

a

Fig.!. a) Cut-away viewof rniniature modularunit. A complete logiccircuit is here mountedon a phenolic board withprinted wiring; two suchboards are contained inone modular unit. Thelength of the block isabout 5 cm. b) a series ofunits as in (a), mountedon a printed-wiring card.

b

soldering. If necessary, the units thus obtained couldbe protected against climatic influences by encapsulat-ing them in plastic.

If the connections between these units are also pro-vided by printed-wiring cards, a very compact con-struction is obtained. This technique has already beenin common use for more than 5 years, and is known asthe miniature technique (fig. la, b).

We are now however in the middle of a spectacularfurther development as regards miniaturization. Minia-turization has always been an aim in the electronic in-dustry. In some cases it was an end in itself, e.g. for air-borne equipment; in computers it is also a means of

hopes that the smaller circuits will give higher switch-ing speeds.

We may distinguish two basically different methodsfor making these integrated circuits, viz the thin-filmtechnique, based on the methods used for making re-sistors and capacitors, and the monolithic technique,which is based on the modern planar technique usedfor manufacturing transistors.

In the thin-film technique, a number of resistors andcapacitors, together with the necessary connections,are evaporated on to a glass substrate in the form of athin film. The output contacts of these units are alsoevaporated on to the glass. One difficulty connected

1965, No. 3 DEVELOPMENTS IN ELECTRONIC COMPUTERS

with this technique is that the transistors and diodesneeded must still be soldered on to the configurationin one way or another. Larger units are made e.g. bypiling a number of plates together (fig.2). Thesemicrocircuits allow the packing density to be increased

a

pectation has not yet been realized, but hopes are stillhigh that the prices of these circuits will fall so that in afew years they will be able to compete with convention-al circuits.

It cannot yet be predicted which of the two above

71

b

Fig. 2. Microminiaturization. (I) Microcircuit on glass substrate. Length 3 cm. The circuitshown (one stage of a shift register for 100 kc/s) contains two transistors, six diodes, 10 resis-tors and four capacitors. b) A number of microcircuits stacked together to form a larger unit. Inthe background may be seen an equivalent unit in the normal minlature technique (the boardshere are not encapsulated to form modular units as in fig. I).

by a further five times, to about 15 components per cm''.In the monolithic technique, the diodes and transis-

tors are made very easily by forming them by the planartechnique in a silicon crystal; but here it is the resis-tors and capacitors which are not so easy to make. Thistrouble is got round by using as a capacitor a P-Njunction biased in the reverse direction, and as a resis-tor a diffused strip of a semi-conductor materialofappropriate conductivity, insulated from the rest of thecrystal by a P-N junction biased in the reverse direction.In principle, this technique can give much greaterpacking densities than even in the microcircuits; if weestimate the improvement in packing density as beingincreased by a further factor of 5, we certainly are on thesafe side. The factor thatcan be obtained depends main-lyon the art of connecting these little "chips", whichonly bave a surface area of a few mms, with miniatureleads to otber units (fig. 3).

By 1960, this technology was so far advanced thatthe manufacturers (in the first place Texas Instru-ments and Fairchild) saw that microminiaturizationmight well become an economic proposition. This ex-

mentioned techniques will finally WlI1 the race. If itproves possible to evaporate still smaller resistors andcapacitors with narrow tolerances on to the monolithiccircuits, this will give very elegant units. However, apossible factor in favour of the thin-film technique isthe fact that a method has now been developed formaking field-effect transistors and diodes (the Schottkydiode) by a thin-film technique too.

There are still practical problems in connection withthe applications of these small circuits. There is thequestion of the heat dissipated, which imposes a limiton the packing density. Further, we have the above-mentioned problem of reliably connecting up a numberof these small units with the aid ofminiature techniquesThis would allow larger functional blocks to be made,which could then be combined by conventional means(printed wiring, plugs and sockets). In this connection,the question immediately arises as to how many unitsshould optimally be included in one block. Withoutgoing into details, we may say that here a number ofcontradictory factors, which are partly dependent onthe state of the art, play a role: bigger blocks may be

72 PHILlPS TECHNICAL REVIEW VOLUME 26

cheaper to manufacture, but the smaller a block, themore easily can one dispense with the demand that itshould be repairable; in this connection the price ofthe block, the life and reliability of its components,the costs of skilled maintenance personnel and the priceof the necessary spare units all play a part. Since partof the micro-logic must be used in combination withconventional equipment in the memory and peripheralequipment of the computer, additional demands aresure to be made on the assembly method chosen.

most unfavourable conditions ("worst-case" calcula-tions), then it is found that no single version of thecircuits can be optimal in al! respects, so that one isforced to make a choice, retaining those propertiesthat are most valuable for the intended application.

The above-mentioned calculations are nowadaysnormally carried out on an electronic computer, thetransistor characteristic normally being approximatelyrepresented by a straight line. A few attempts have al-ready been made to take into account the non-linear be-

Fig. 3. Microminiaturization by the crystal technique. The little square in the middle (a "chip"I X I mrnê) contains [our transistors and four resistors. The connections leading to normal con-tacts placed round the chip mayalso be seen.

The whole field of microminiaturization is under-going a revolutionary development at present, and aswith all revolutions it is very difficult to predict whatwill be the final outcome.

We will close this section with a few remarks on thetypes of circuit which are used in computer sub-assem-blies. The most common are the "nand" and the "nor"circuits. These can be realized in various ways, as maybe seen from jig. 4, which shows the most usual solu-tions [5l. Many attempts are being made to find an op-timum solution for these and various other circuits.Many different criteria can be used in this connection-speed, sensitivity to interference, the generation of in-terference, the influence of tolerances and drift of corn-ponents, energy consumption, attainable "fan-out"(i.e. the number of similar circuits which can be con-trolled by the circuit in question). If however one con-siders how the possible circuits would work under the

haviour of the barrier layers in these circuits, whichallows in particular a more accurate quantitativedescription of transient phenomena. It may be notedthat electronic computers are being used more andmore for developing better computers and for the im-provement of the various computer-manufacturingprocesses, e.g. for the making of optimum wiringpatterns, the making of drawings, parts lists and wiringlists, etc. - thus contributing to a kind of eugenics ofthese robots.

High-speed main memories

The most important contribution to the improvementof the reliability of electronic computers has doubtlessbeen the introduction, some 10 years ago now, ofmagnetic core memories. The memories used until thencontained acoustic delay lines usually with mercury(but sometimes with nickel wire or quartz) as trans-

1965, No. 3 ' DEVELOPMENTS IN ELECTRONIC COMPUTERS 73

mission medium, or cathode-ray tubes, usually calledWilliams tubes after the inventor of this application.The drum memories, which in those days were alsosometimes used for main storage in slower machines,will be discussed in the following section. Both of theother types of memories mentioned had serious dis-advantages. For example, the temperature dependenceofthe delay lines was a problem; in the Williams tube,the problem was the poor persistenee of the chargepattern on the screen, which necessitated continualregeneration.

IRTL I +11>

RT R3A

RTB

C A

R2

"'i?

I OTL I +~

RT.

A

B R2C

R3

-11> -

for the decay of the transient phenomena in the ampli-fiers for the read currents and in other parts of theelectronic circuit. In order to obtain shorter cycletimes,one must try to limit the transient phenomena as wellas to reduce the switching time proper. For the latterpurpose, wemust use cores with a higher coercive forceand use a higher field strength for reading and writing.This means, however, since one wants to obtain thehigher field strength with roughly the same currents,that one has to use smaller cores. This allows the mem-ory matrices to be made more compact and since in a

+11>

+~

Fig. 4. Various logic circuits using transistors. The top two are "nor" circuits, and the bottomtwo "nand" circuits. The notation A +B+C denotes "not (A or B or C)", i.e. the value "0"(in this case represented by a low voltage) will appear at the output as soon as the value" 1" (ahigh voltage) is present at at least one of the inputs ABC. The notation ABC denotes "not(A and B and C)", i.e. the value "0" appears at the output only ifthe value "1" is present at AandBandC.

Just when the vanous technological problems ofthe Williams tube had been more or less solved, themagnetic core, made possible by the development offerrites with a square hysteresis loop, came on to thescene, thus inaugurating a period of great and rapidprogress in memory techniques.If we disregard certain types of larger core for spe-

cial applications, we may say that the first core whichfound practical application in computer memories hadan external diameter of 0.080 inch i.e. about 2 mm.This made possible a cycle time of 10 !LS.The cycle timeconsists of two switching times (for the destructivereading and for the writing back of the information)and a certain time between and after these two, needed .

smaller matrix the voltage pulses occurring duringwriting are smaller, this also has a favourable effecton the transient phenomena in the amplifiers. Forthese various reasons, the cycle time could be reducedto 6 !LS by use of a core 0.050 inch in diameter (fig. 5).The development workers then concentrated mainly

on improving the reliability. Better knowledge of thesintering process of the ferrite used allowed the pro-perties of the memory element to be made much morereproducible. A better choice of the composition ofthe material gave less temperature dependence and a

(5) R. Foglesong, Integrated circuits design and application,Semicond. Prod. and Solid State Techno!. 7, No. 3, 32-34 and39-42, 1964.

74 PHILIPS TECHNICAL REVIEW VOLUME 26

the cores on which these values weremeasured differed appreciably as re-gards both the thickness and the ferriteused, while the currents used also varied,as may be seen from the table.

Many attempts have been made tofind a better alternative to the magneticcore in high-speed memories, e.g. bygiving the ferrite another shape, bylooking for other materials with a squarehysteresis loop and by use of materialswith a basically jas ter switching mech-anism.

For example, it has been proposed tomake a memory of fiat square sheetsofferrite containing a matrix of holes [61.

Part ofthe wiring is automatically fixedon to the sheets, while the rest canvery simply be threaded through theholes. To the best of our knowledgethe only large-scale application of thesesheets is in the memories for the above-

mentioned electronic telephone exchange at the BellLaboratories.

Fig. 5. Ferrite cores 0.050 and 0.030 inches in external diameter. The match-headindicates the scale.

squarer hysteresis loop. Much attention was also paidto improving the methods of threading the cores in thememory matrices.

The steady improvement in the mastery of the tech-nology allowed even smaller cores to be put into use in1961, the external diameter being reduced to 0.030inch (fig. 5). As a result of this, and of the appreciablespeeding up of the associated electronics which hadbeen realized in the meantime, the cycle time was re-duced to about 2 [LS.

It goes without saying that the problems ofthreadingthe cores became greater: at least three wires had to betbreaded through a hole of diameter 0.4 mm. Thegeneral opinion was that this represented the practicallimit of these memories. Recently, however, it has prov-ed possible not only to make still smaller cores (exter-nal diameter 0.020 inch), but also to thread these witb-out too much difficulty (fig. 6 and 7). These cores givecycle times of about 1 [Ls.

The fundamental and practical limitations of thistechnique are now being examined. There is no doubtthat still smaller ferrite cores can be made, but it is notyet known whether it would still be possible to threadthem, and if so whether other difficulties will not thenarise, e.g. that the resistance of the thin wires mightbecome excessive. It is however to be expected thatcycle times of about 0.5 [LSwill be possible within a fewyears. Table lIJ gives the switching time for the above-mentioned cores, together with the necessary switch-ing current and the year of introduction. One shouldbeware of drawing too stringent conclusions from thedifferences in switching time given in this table, since Fig. 6. Part of a memory matrix with cores 0.020 inch in diameter.

Table Ill. Dimensions, switching time and switching current ofsuccessively used magnetic memory cores.

External Year of Switching Switchingdiarn. of care introduetion time current

(inches) (fLS) (mA)

0.15 1953 10 400

0.080 1955 1.5 700

0.050 1958 1.0 500

0.030 1962 0.5 650

0.020 1964 0.2 850

1965, No. 3 DEVELOPMENTS IN ELECTRONIC COMPUTERS 75

Fig. 7. Threading a memory matrix such as that of fig. 6.

Some dielectrics, e.g. barium titanate and GASH [7l,

have a roughly square hysteresis loop, but one prop-erty makes these substances inferior to the magneticmaterials, viz, the fact that the memoryelements basedon them have two poles. With the magnetic materialsone can make elements with separate input and outputwires, which gives many more possibilities. Anotherdisadvantage in e.g. barium titanate is the memoryloss.

The most important memory element with a fastswitching mechanism is the thin magnetic metal film(fig. 8), where the magnetization is not reversed bymovement of Bloch walls but by the much faster ro-tation of the direction of magnetization in a Weiss re-gion. The switching times which can be obtained in

(6) J. A. Rajchman, Proc. IRE 45,325, 1957.R. H. Meinken, Conf. Magnetism and magnetic materials,Boston 1956, p. 674.J. A. Rajchman, Computer memories: a survey of the state-of-the-art, Proc. IRE 49, 104-127, 1961.(7) Guanidine Aluminium Sulphate Hexahydrate. - Guanidineis a substance which was first discovered in guano.[8) R. Shahbender, C. Wentworth, K. Li, S. Hotchkiss and J.Rajchman, Laminated ferrite memory, AFIPS Conf. Proc. 24(1963 Fall Joint Computer Conference), pp. 77-90.

this way are ofthe order ofnanoseconds, but no memoryhas yet been made which takes full advantage of thisspeed. The only thin-film memories on the market atpresent have cycle times of from 0.3 to I fLs,while in alaboratory study a prototype of a memory with a cycletime of 0.1 fLShas been realized. Thin films also havethe disadvantage of a relatively very low output volt-age, viz, about 1 mV, compared with tens of mV in themagnetic cores. The ferrite core is thus still leading forthe moment, and it looks as if the thin films will ratherlose than gain ground.

This does not mean that the thin film has alreadylost the race. It is conceivable that good technologicalmastery of the manufacturing process of the thin-filmmemories (including their control and read circuits)could eventually make these memories cheaper than

Fig. 8. A memory in which each element is formed by a very thinmagnetic metal film on an aluminium carrier. The pencil point(which is reflected in the aluminium) indicates the scale.

The wiring for this memory is applied to various Mylar foilswhich are laid on the pattern of elements.

corresponding magnetic-core memories. In this case,the price rather than the speed would be finally decisive.So far, however, this is not the case. And whether thethin film will still stand a chance in a few years' timewill strongly depend on the progress made with thecryogenic memory, while the newest inventions in thefield of ferrite memories, the microferrites and thelaminated ferrites [8] also appear very promising.

Cryogenic memories make use of substances whichbecome superconducting at low temperatures. With

76 PHILlPS TECHNICAL REVIEW VOLUME 26

thin-film techniques it is possible to make very smallmemory matrices on this principle (e.g. 128 X 128 bitson a surface of 5X 5 cm-). The circuits for the selec-tion of the desired word from the memory can be madeby similar means and form an integral whole with thematrix. Memory capacities of the order of 109 bitswould seem to be possible in this way. However, thevery low resistances limit the switching speed of largerunits, which is determined by Llr relaxation times. Theswitching time will be of the order of 1-10 fJ-s.One ad-vantage of these cryogenic elements is that, thanks tothe infinite ratio between resistances in the normal andsuperconducting state, no parasitic voltages are pro-duced by the coincident currents i [91 used for switching,so that one can in principle make very large memorymatrices. There are still plenty of technological pro b-lems, however. The problem of finding the most eco-nomical cooling method for this application must alsobe considered uojIt is hoped that microferrites and laminated ferrites

may offer possibilities for memories of the order of 107bits. Cycle times of from 0.1 to I fJ-Shave been meas-ured on a number of prototypes of this kind. The out-put voltages are of the same order of magnitude aswith the metallic magnetic films. The read currentsneeded can vary from 50 to 200 mA.

All these figures show that memories, like the logiccircuits, are tending more and more to a microminia-ture form, so that we may in the long run expect muchsmaller computers, where the logic and the memoryare better matched and thus have a more integratedcharacter than is the case at present. The increasinglynoticeable attempts to put selection circuits in thememories themselves also work in this direction.

Apart from the high-speed main memories discussedin this section, there is also a need for a fast, inexpen-sive permanent or semi-permanent memory, from whichthe information can be non-destructively read veryrapidly, while changing the memory contents is relativ-ely slow (e.g. by photographic processes). The demandfor memories of this kind will increase more and morein connection with the steady increase in the amountof permanent information which one wishes to feed toan electronic computer in the form of translation pro-grammes, sub-routines etc. Another reason for thedemand for nondestructively-read memories is the riskthat in the normal proeed ure. i.e. destructive readingfollowed by writing back of the information in question,a disturbing pulse may change the contents of the mem-ory. This is catastrophic for memories for use in spaceflight, but even in telephone exchanges it is highlyundesirable, since it could result in a subscriber losingthe correct number as a result of a permanent change inthis number during writing back in a memory.

No completely satisfactory solution for memories ofthis kind has yet been found. Various methods havebeen worked out, but they all leave room for improve-ment. For example, in the telephone exchange whichhas already been mentioned several times, the BellLaboratories used the twistor as a semi-permanentmemory element for storing permanent subscriber dataand programmes for the connecting of one subscriberwith another. Growing interest also exists at present inthe "Biax" element, one of the several multi-hole ferriteelements which have been developed (fig. 9), in con-nection with its use in a nondestructive-read memorywhich can be read quickly (about 100 ns) and can still bewritten fairly quickly (of the order of a few us).

Fig. 9. A pile of "Biax" elements for a semi-permanent memory.Each element contains two holes at right angles la one another.The length of the element is about 2 mm.

For the sake of completeness, we may briefly men-tion here the reappearance of the delay line, in twoforms, viz, the polygonal form in glass with a very lowtemperature coefficient and the magnetostrictive delayline [111. Owing to the great increase in the speed of thelogic circuits, these delay lines can, when used in nottoo large serial machines, provide the key to reasonablyfast, and at the same time cheap, solutions.

Slow memories: drums, discs, magnetic tape

The need for larger memory capacities than can eco-nomically be provided by the fast main memories wasalready felt in the first electronic computers. The priceper bit is here the decisive factor. Here too, magneticrecording still reigns supreme as regards simplicity andreliability, and this state of affairs may be expected tocontinue for years, although laboratory investigationsare now being carried out on a number of interestingdiscoveries, e.g. thermoplastic recording. In fact, thismethod is probably more suitable for use in a semi-per-

1965, No. 3 DEVELOPMENTS IN ELECTRONIC COMPUTERS

Fig. 10. Magrietie-tape units, used with the COC 3600/3200 computer in Philips ComputerCentre, Eindhoven.

manent memory, since the writing rate is on the lowside.

Table IV gives some data about a number of represent-ative memory types which will be briefly discussed inthis section, to show how the present state of affairscompares with that of about 10 years ago.

Table IV. Typical data of magnetic tape, drums and discs in 1954and 1964.

1954 1964

ape:

Reading and writing speed (bits/s pertrack) 12000 170000

Writing density (bits/mm) 4 60

Start-stop time (ms) 10 2-3

Drums:

Mean access time (ms) 10-20 10-20

Writing density (bits/mm) 2-5 26-40

Capacity (bits) 105_106 107-108

Discs:

Mean acces time (ms) 600 20-200

Writing density (bits/mm) 2-4 20-40Capacity (bits) 108 108-109

Magnetic cards:

Mean access time (ms) - 100-600

Writing density (bits/mm) - ca. 10Capacity (bits) - 107-1010

Magnetic tape is still much the cheapest informationcarrier, with a price of the order of 0.0003 dollar perbit. The writing rate for tape has increased by a factorof nearly 30 in these 10 years. The rate of 170 000 bits/sgiven in the table refers to the IBM's latest develop-ment, "hypertape" .

Meanwhile, the magnetie-tape field has been under-going a tacit standardisation of recent years: a numberof firms have been marketing tape units which candirectly read the tapes written by IBM machines, andvice versa. In this connection, the tape speed has beenstandardized at 75 and 150 inch/s and the writing densi-ty at 200, 556 and 800 bits/inch.

A great problem of a mechanical nature is the quickstarting and stopping which is desired in order to leaveas little unwritten tape as possible between blocks ofinformation. Normally one needs a time of 7 ms forstarting and stopping; modern techniques - whichstill however make the tape units rather expensive-can achieve times of 2 to 3 rns (fig. la).

[9] For the concept of coincident current in the writing and read-ing processes, see e.g. H. J. Heijn and N. C. de Troye, Philips tech.Rev. 20,193-207,1958/59, in particular p. 199.[10] See also G. Prast, A gas refrigerating machine for tempera-tures down to 20 OK and lower, Philips tech. Rev. 26, 1-11, 1965(No. I).[11] For ultrasonic delay lines see e.g. C. F. Brockelsby and J. S.Palfreeman, Philips tech. Rev. 25, 234, 1963/64 (No. 9).

77

78 PHILIPS TECHNICAL REVIEW VOLUME 26

The fact that magnetic tape entails a very high accesstime (ofthe order of seconds to minutes) owing to thefact that the information is written serially all alongthe tape has stimulated the demand for "random-access"memories, i.e. memories which can produce any des-ired piece of information without appreciable delay(appreciable, that is, for the application in question).The development here has been mainly in two direc-tions; drum memories and disc memories.

We have already mentioned in passing the fact thatnot so long ago (about five years) some relatively small,slow computers still used drums as their main memories.Now this would be quite out of the question: the

a

magnetic cores have advanced so far the question nowarises whether they might not even provide an econo-mic solution for the high-capacity memories for whichdrums are now used. For the moment, however, thedrum still has a place as a big secondary memory witha capacity of the order of 107 to 108 bits and an averageaccess time of about 10 ms (fig. 11).

Of recent years it has proved possible to increase thewriting density on the drum considerably by a new wayof mounting the read-write heads. Things are so ar-ranged that the heads float stably on a thin cushion ofair which rotates with the drum. This has allowed thedistance between the heads and the surface of the drumto be reduced to a few microns. A further improvementin the drums is the saving in electronic reading andwriting equipment obtained by providing means forshifting the heads mechanically over several tracks.This idea is in fact more than 10 years old, but it was

difficult to realize properly without the floating-headtechnique.

A curious hybrid of the tape and the drum is the"carrousel tape" brought out by Facit. In order to re-duce the access time, the tape is divided into 64 smallpieces, each about 10m long, each of which has itsown little reel. The carrousel with the 64 reels mustfirst turn until the desired reel is in front of the readingdevice, and then the tape of this reel must be fed pastthe reading device. The average access time is here 1 to2 s and is thus a factor of 100 less than in normal tapeunits.

Disc memories, which came on to the scene nearly

b

Fig. 11. a) Memory drum with drive mechanism and read-write equipment. In b) one of the covers is removed andthe hinged frame with read-write heads is lifted so that alittle of the drum can be seen.

10 years ago (Ramac 355), have undergone improve-ments similar to those of the drums, viz, an increase inthe writing density and in the capacity. A special de-velopment in this field is the disc memory for theIBM 1440 computer, where a complete packet of discscan be exchanged. This thus gives a theoretically un-limited memory capacity, just as with magnetic tape,but with a much better average access time and theflexibility is much increased.It mayalso be mentioned that some firms (IBM,

NCR, RCA) have tried to increase the flexibility anddecrease the access time in a big memory by slicing upthe magnetic medium to give magnetic cards.

An important question which keeps on cropping upis whether there will still be a place for magnetic tapememories after the introduetion of these big random-access memories. It has been suggested that one willprobably still keep the magnetic-tape memory as a sort

1965, No. 3 DEVELOPMENTS IN ELECTRONIC COMPUTERS 79

of spare memory, into which the disc memories can be"emptied" before being used for new purposes. Tapecan also be used in order to provide a new startingpoint in case an error should occur in the machineduring calculation which would make the informationcontained in the disc memory unreliable. With thisconception, one need no longer make high mechanicaldemands on the tape units; e.g. short start and stoptimes would no longer be required. This allows thewhole unit to be made much cheaper.

The possibility of having at one's disposal memoryunits in blocks ofabout 109 bits with access times ofO.lto 0.2 s has given many people the idea of making abig computer with a very big memory, which byanal-ogy with a telephone exchange could be used as asort of information exchange, connected via a commu-nication network (data transmission network) with anumber of subscribers. The subscribers could be pro-vided with equipment varying from simple keyboardsto complete satellite computers, with the aid of whichthey could obtain access to and make use of all the com-puting and memory facilities of the exchange.

It is a fact that in America it is expected thatthe telephone network will undergo considerableexpansion for the purposes of data transmission. Ittherefore seems to us that the information exchange isa real proposition. Computer techniques will then how-ever have to make more use of redundancy than hasbeen done in the present designs, in order to give thesystem as a whole the reliability which is so necessaryfor this plan. The operation of such an exchange willalso require an enormous amount of initial programm-ing work.

Peripheral equipment

In order to give an impression of the progress madein the field of peripheral equipment over the past tenyears, Table V gives various important data about themost used input and output units. Inspection of thesedata will show that a much greater advance has beenmade in the input speed (by a factor of 20) than in theoutput speed (a factor 6). The reason for this is that theoutput units are subject to mechanical limitations,which can be eliminated in the input units by the useof optical systems. Here too there is a mechanicallimit

Table V. Speeds of various types of input and output units in 1954and 1964.

1954 1964

Punched-tape reader 100 2000 characters/sTape puncher 50 300 characters/sPunched card reader 2 50 cards/sCard puncher 5 cards/sLine printer 2.5 15-20 lines/s

in the last resort, but this is related more to the hand-ling of the information carrier (punched cards orpunched tape), than to the reading mechanism. As isknown, the tensile strength of paper tape limits thereading speed to about 4000 characters/so There are anumber of tape readers on the market for 1000 char-racters/s, so that not much more progress is possiblehere (jig. 12). Speeds of 2000 charactersjs have been

Fig. 12. Commercially available punched tape reader with a read-ing speed of about 1,000 characters per second, in use in Philips'Computer Centre, Eindhoven.

reached in laboratory set-ups. The punched card is alittle stronger than the paper tape, but here too thepresent-day results lie quite close to the physical limit.If one wishes to achieve greater speeds with the out-

put units, one will have to make use of completely differ-ent principles. Investigations are being carried out one.g. a method in which the punching is not done with amechanical punch, but by means of the pressure wavecaused by the passage of a spark [12l. A similar methodshould be able to form the basis of a high-speed prin-ter. Another possibility is to replace the punching by

[12) See G. Haas, Problems and trends in the development ofperipheral equipment for computers, to be published in Philipstech. Rev. 26, 1965 (No. 4/5/6).

80 PHILIPS TECHNICAL REVIEW VOLUME26

the application ofblack marks, but this has not proved. very reliable so far, the margin between light and darkofthe reflected light being rather small. The xerograph-ie methods, which one would like to develop for out-put units, are also unreliable, but in another way: it isimpossible to guarantee the production of a number ofidentical copies. An identical error in all copies is lessobjectionable than a good first print followed by otherswhich might contain an error, or vice versa.Itmay be asked whether it is really worthwhile spend-

ing an effort on the investigation of further improve-ments to this kind of input and output equipment, nowthat methods for the automatic reading of typed orprinted matter are on the way.Banks are already standardizing a method for the

reading of stylized magnetic characters (type E 13B);see fig. 13. This standardization began in the UnitedStates, and has been followed by Canada, Australiaand Great Britain. Other suggestions have been madein Western Europe (type CMC7 in fig. 13); it is not yetcleat how things will develop in Europe in thisrespect [13l.

A great deal of development work has already beendone on optical character readers, which will ultimate-ly have a much wider range of applications. One ob-stacle to more general application at present is a lackof standardization of the types of letters to be read [14l.

However, the IBM has already included an opticalreader as direct input for the 1401 computer. Thishas a reading speed of 480 characters per second, andcan deal with up to 400 documents per minute. Thismay possibly be taken as a standard to be followed forthe moment by other computer manufacturers.Will these optical readers perhaps change the entire

character of the peripheral equipment - apart fromthe line printers which will still be needed? Or willthere still be a place, albeit more modest, for the good

[13) w. J. Bijleveld, Automatic reading of digits, published byStichting Studiecentrum voor Administratieve Automatisering,Amsterdam, March 1963. '[14] For proposals for the standardization of the reading ofdigits made by the Dutch PostalOrder Service see: W. J. Bijleveldand A. J. van der Toorn, Methoden voor het met de hand invul-len van automatisch te lezen getallen (Methods for writing byhand numbers to be read automatically; in Dutch), Ingenieur 76,A 693-702,1964 (No. 46).

111111111111,111 IIII111111

11111

111111111 I1

111 IIII "In 11

1111111III1

'11 hI1111III

IIIIIIIII

1111111

III I111111

11::: I'1111

b?SCJOFig. 13. Stylized magnetic digits which can be visually recognizedand can also be read automatically by a scanning process, Abovetype CMC 7, below type E 13 B [131.

old punched card as a very durable and flexible memorymedium? We will not venture to make any predictionsabout this, but we do expect that within a few years aclear answer will be possible to these and many otherquestions which have remained open in this survey:the rapid development of the computer, which we re-ferred to at the start of this article, is still progressingat full speed.

Summary. This article is based on a lecture given by one of theauthors (v.d.W.) to the Dutch Computer Society. The develop-ments discussed, which are to a certain extent still in full flight,concern 1) the improvement of the reliability, coupled with therealization of a more compact construction and higher switchingspeeds. As essential steps in this process are described the minia-ture technique and microminiaturization in "integrated circuits",in which transistors and diodes are made in one unit, togetherwith resistors, capacitors and connections; 2) the realizationof larger and larger memories, preserving a reasonable access.time. The gradual reduction in the size and increase in the speedof magnetic memory cores and the successive new techniques(thin-film, etc.) are discussed in some detail; 3) the speeding upof the input and output of data. These processes are subject tomechanicallimitations, and the progress here has so far been thesmallest.